Small Update:

Driftland: The Magic Revival
Eagle Tamer, Human Adept and Legendary Explorer can be achieved since latest patch.

Pathfinder Kingmaker
Honest Looting can be achieved but I had to roll back to version 3.0.3

Looking into Galaxy's data, the following games have GOG Galaxy achievements in the database, but are apparently not active yet, for whatever reason, likely difficulty on the dev's side in implementing them?
AI War 2 - Not on the 'missing achievements' list, but not advertised as having any
Albedo: Eyes from Outer Space - Only 1 achievement implemented, the game is also missing some patches
Anna's Quest
Brigador: Up-Armored Edition
Celestian Tales: Old North - Originally listed with achievements, but they couldn't be unlocked and achievements were deactivated
Crossing Souls
Eador: Masters of the Broken World - Only 1 achievement implemented
Eisenwald: Blood of November
Fallout 3 - Especially interesting considering it doesn't even have achievements on Steam
HYPERGUN
Infectonator 3: Apocalypse
Into the Stars
Layers of Fear
Legends of Eisenwald
LIMBO
Machinarium
Mana Spark
Nantucket - Not on the 'missing achievements' list, but not advertised as having any
Obduction
On Rusty Trails
Owlboy
Quest for Infamy - Only one or two achievements implemented
Samorost 3 - Only partially implemented
Shag-Fu
Slime Rancher
Steel Rats
Tesla Effect - Partially implemented, not implemented on Steam either
The Inner World - Partially implemented
WARTILE
What Remains of Edith Finch
Witcher 2
Ys I & II Chronicles - Briefly advertised as having achievements, but they weren't attainable

GOG going the extra mile and adding achievement support to games that don't even have it on Steam would definitely be an interesting course of action. It would certainly give them an upper hand in the eyes of some people.

Thank you, hopefully they can fix them after the holidays.
I keep it under observation, it seems to work for other people, maybe it's a problem specifically related to Galaxy 2.0.
Thank you, I have updated the list accordingly.

A very interesting list, I didn't know it was possible to identify even deactivated achievements. Many of the games on the list correspond to what I already knew about various games whose achievements have been abandoned. I will analyze the list in more detail soon.

AI War 2 if I'm not mistaken has added the achievements on Steam only recently, so with a bit of luck they are working on it to implement them on GOG too.

Little Big Workshop is missing the achievement "Lazy Pete". It was added on Steam on December 5th.

Zwei the Arges Adventure

The achievement is "True Treasure Hunter" and to unlock it you have to collect all the other achievements..I already unlocked the other achievements but this one didn't unlock.

Thanks, Little Big Workshop added to the list.

Thanks, Zwei: The Arges Adventure added to the list.
I'm starting to wonder if there are any XSEED games without broken achievements XD

Golem

The achievement "Home Wrecker" is broken (also on Steam).

I've had no luck unlocking any of the achievements in The Great Perhaps. The game card doesn't advertise them, but they are visible in Galaxy, maybe they aren't fully implemented yet.

Thanks, Golem added to the list

Thanks, The Great Perhaps added to the list. I had personally asked to add the "achievements" tag because I had seen the achievements appear on Galaxy, I guess they removed it again because they don't work... sigh.

Have you ever tried to contact Kalypso Media Digital customer support? I'd like to try contacting them with a list of all the broken achievements of their games but it would seem that including the save game might help them find the problem.

Maybe i've tried, but not so sure right now.

Save files:
https://sharefiles.app/download/c4b25f97fff19070ae10facba92baa2133c34f1b

Thank you, Vikings Wolves of Midgard also has in-game achievements?

Nope, only the Galaxy ones.