[EDIT* I wrote this right after finishing the game and getting the bad ending. Looking back now, I think that ending was exceptionally well written and executed (as sad as it was). I do not think it is a 'wrong' ending or failure on the part of the player; it is simply an ending, and for anyone who got it, it should be as valid as the other two possibilities.
Never the less, I decided to keep what I wrote for anyone who would want to bother reading it, since I do still believe that the relevant choices are too unclear and most can be justified both ways. I think it says a lot of good about these three games that the ending has such a lasting impact on so many people.]

This is something that I have seen many people complain about.

I do not think the Ciri dialogues were fairly implemented, especially considering how brutally harsh the outcome may be. The dialogues that are perceived, by the player, as the important ones have no effect on Ciri's outcome. The dialogue choices that do have an effect are unobvious and most do not have a 'good/bad' binary, or even a 'positive/negative' binary. You will often have to choose between two good options.
The true binaries here seem to be 'mature/childish' and 'supportive/unsupportive', and the 'childish' and 'unsupportive' options end up being the correct ones. There are two that have a 'sensitive/insensitive' binary (one of which is optional), and therefore their choices are clearer. In fact, everyone whom I saw posting about this, who got the bad ending, chose the 'correct' dialogue options in these instances.

Obviously they were going for a father and daughter relationship, with the idea that we as parents must let our children grow up and live on their own. But if they were doing that then, in my opinion (and in the opinion of several others that I have seen), several choices need to be reworded or even flipped.
And once again, it needs to be said, that these are the innocent 'in between' moments, not the major life-decision moments. For example: when speaking with the lodge, it is not about saying 'Ciri can decide for herself' (treating her like an adult), no, that has zero impact. Instead it is about accompanying her.
The issue is further compounded by the fact that the dialogue options in the game, in general, are unclear in terms of their result. There is no indication, for example, that Geralt would give Ciri advice on how to talk to Phillipa and Margarita. If the dialogue options more closely matched their outcomes, or matched them at all, I believe it would have helped many players tremendously.

Choices that many thought would have a major impact on Ciri, turn out not to have any impact at all:
Telling her to either stay safe in the fortress or do what she wants - no impact.
Running after her like a hovering parent or let her deal with her own grief after Vesemir's death - no impact.
Telling her that she is not good enough to fight Imlerith (who she wanted to fight) - no impact.
Choosing one of several dialogue options after fighting the crones, like 'coming here was a good idea' - no impact.
Having a heartfelt conversation with her around a fire - no impact.
Stealing horses with her or not - no impact.
Saying which way she should go, or saying she can make up her own mind, with the Lodge - no impact.
Telling her to either stay ashore against her will or do as she likes in the final battle - no impact.

The things that do have and impact:
Have a heartfelt talk as equal adults over drinks / Have a snowball fight
"Incorrect" choice:
Not choosing the lighthearted option because that would be insensitive and would downplay her situation. By saying that she need not be good at everything Geralt is admitting that he also is not good at everything. It is a mature realisation for any of us. To ignore the weight on her shoulders and tell her to have a snowball fight instead is immature and inconsiderate.
"Correct" choice:
Take her mind off things by having a snowball fight.

Accompany her to speak to the Lodge / Tell her to go by herself and give her advice
"Incorrect" choice:
Go with Ciri when she talks to the Lodge. She is clearly stressed, so you can either go with her to provide moral support, or give her advice, but either way leave the decisions up to her. So you choose to just be in the room to give her some confidence. The same as you did when you went with her to her father (if you chose to). We know that sometimes someone we care about simply needs us to be there with them. Besides, Geralt was coming to speak to the Lodge anyway and all current decisions of the mission pertain to Ciri. Being present at this juncture makes complete sense.
"Correct" choice:
Tell her to go into the lions den by herself. (No indication that Geralt was going to give her advice on how to speak with the sorceresses. Therefore the only 'moral support' option seems to be to go with her)
(The flashback of this scene seems to indicate that it is linked to what Geralt said during the meeting, not whether he went with her)

Tell her to calm down / Trash the lab
"Incorrect" choice:
Do not tell her to destroy the lab because losing control of your emotions like that is not something and adult should do. Also, breaking someone else's property and hard work just because they did not share all information with you is not justifiable. Even though Geralt dislikes Avalach and would not mind seeing his property destroyed, he is an adult and chooses restraint and does not use his daughter's emotional state to fulfill any such possible desire. Also, Avalach and Ciri are at least partners, if not friends, and we would not destroy a stranger's property under such flimsy motivations, let alone someone we know and respect.
Also, make a heartfelt gesture by giving her Lora's amulet.
"Correct" choice:
Have an embarrassing temper tantrum that the only remaining adult in the room cringes at with incredulity. (This dialogue option initially seemed to imply that she might use her powers to completely destroy the workshop)

The option to accompany her to bury Skjal is the only one with a clear moral choice that may have meaningful consequences. Obviously any considerate person would go with her.
If you chose to take her to the Emperor, telling Emyr to keep his money is also more obvious and honourable, and probably the choice most people will make. Though most would probably not even take her to Emyr.

Thinking like a rational adult, and treating Ciri as an adult are the wrong decisions.
The problem then is that you can not make a clearly bad decision or clearly good decision to indicate to the player which direction Geralt and Ciri's relationship is taking. Moreover there is no indication of strain in their relationship to show the direction that this is going.
The thing is that if you do not make an internet search you may never realise that these moments were the ones that decided her fate. Even looking at her flashbacks may not be sufficient, and could result in many hours worth of trial and error to get a good ending (an endeavour which would obviously undermine the emotional impact).

The very real problem here is that it happens at the end of a very long game which is the third and final installment of a very long franchise. Having such innocuous choices lead to such a terribly depressing ending, especially if the player was careful to make what they perceived as the 'right' decisions, is not fair. For such a dismal outcome at the end of a game where players invested so much time, effort and emotion, the intentions or results of dialogue choices must be made clear.
There are people who adored this series and this game, who disowned it after that ending. Some might say that they are overreacting, but it just goes to show how much investment players have in this game. Having such an ending after 500 hours (and if you count the other two games 600 - 700 hours total) can result in a fairly strong reaction.

Some have speculated that Ciri is still alive in the bad ending but chose not to come back to Geralt. If that is the case then it means that Ciri is profoundly immature. It means that she looked back on her recent interactions with Geralt and, realising that he would treat her with maturity and not indulge her impulsivity, decided not to return to him. She would make him think she died and leave him to his horrifying grief, which is an act of staggering inconsiderateness.
"Oh, and for the record, letting someone think that someone they love is dead when they're not is quite cruel." - Bill
In no universe does Geralt come within a million light years of deserving such cruelty.

I am not saying that such an ending should not exist, but to have it happen as the result of choices that are so unclear in their impact and seem like good, even innocent choices at the time, is absurd.

I agree about thrashing the lab. I have always though it is not a good way to solve the situation, and I would expect some negative consequences (perhaps if it wasn't so close to the end of the story). But most of the decisions seem to be about letting CIri grow and make her own decisions, even if we don't necessarily agree with them, while keeping her grounded and tied to what makes life worh living.

I can see the point for the rest, and I absolutely stand behind the snowball fight. It was such a heartwarming scene and the standout moment during the flashbacks at the end. If Ciri ever had any doubts about her place in the world, that moment with Geralt will always stick and remind her of her found family.

I prefer that the decisions weren't obvious during gameplay, although I understand how it could be frustrating after a very long game such as this. I myself got a good ending, but if I recall correctly, I was just above the minimum threshold. The Witcher 1 was a lot more obvious: they tell exactly when you are making the decision that determines your ending and from where there is no coming back; no player is going to be caught in the wrong alignment, but it also feels less organic story-wise.

Yes, I agree on it being more organic. Especially since it is not telegraphed to the player which particular dialogues affect the ending. The issue really is that the dialogue options, based purely on their own wording, can be justified either way.

If they had made the wording match the outcome more closely it would have helped. e.g. the dialogue option that says 'Relax. you can't be good at everything' could be understood to mean that Geralt wants to take some weight off Ciri's shoulders. But the actual outcome is him saying to her that she should just give up.
So, the actual actions of Geralt often do not match player intent.

I have seen players, who refused to use a walkthrough, say that they played the game multiple times trying to get a good ending, but ending up with the bad one every time. Eventually they caved and had to look it up.

If the options had been between something like:
1) 'Stop wasting your time with magic'
2) 'Relax a while and gather your thoughts'
Then the intent would have been much clearer, and would have matched their outcomes.

Given that the player is unaware that these particular dialogues affect Ciri in the most profound way and that it is unknown what the actual intent behind the choices are (based on their wording), I think it might have been better if they had a much higher threshold to receive the sad ending. Especially considering how gut wrenching the bad ending is.
I think at this point in the Witcher games' story, the bad ending should almost be more of and easter egg.

I think that the sad ending was very well executed and it should certainly exist. I would purposely play it again some day. But I think it could have been implemented a bit more fairly.

I agree. I think it's intentional, you get the bad ending first, realize there are more endings, then find out what you need to do and play it again. Especially with the DLCs, it's clearly designed with New Game+ in mind, but the game isn't really replayable in general, except if you're going for a different ending and really want the better ending. You can just watch the endings on youtube though if you don't actually want to play through it again just for that.

For what it's worth, I thought the sad ending was the best one, and most likely one considering the world and general tone of the game. The good endings are more like an easter egg/fanservice put in after you complete the intended story (think "true" route for a VN or such).