Has anyone played these? Do they still run and work well on modern PCs?

I haven't played Heroes 7, so I can only speak for Heroes 6 as I tried to play with friends many years ago.

For a singleplayer strategy game Heroes 6 was a quite refreshment to a Heroes franchise. You could reforge castles to your own faction castle, heirloom artifacts and heroes were very strong, etc. But the game could get very annoying during the multiplayer games due to a constant desinc issues. CONFLUX system was very unstable. And I don't know if it's still alive, by the way. I remember it was possible to play multiplayer matches without the CONFLUX stuff (artifacts, heroes) though.But it wasn't that fun, because Heroes 6 is all about CONFLUX stuff: you complete the campaign scenarios - you get new CONFLUX heroes and artifacts.

Knowing ubisoft the CONFLUX servers can shutdown any time. I'd like to check Heroes 6 for you and tell if the game is till alive, but I can't reach the ubisoft connect launcher download page for some unknown reason.

They look like a cheap knockoff from wish.com

HoMM 6 at least has a demo you can try. I found it to be surprisingly good, and a lot of fun, even though I didn't at all like that you could change the faction type of cities.
Unfortunately the game has single player mechanics tied to being logged into uPlay, which to me is a dealbreaker.

I think they should run. Haven't played them but seeing as HoM&M V runs fine, these should too.

As for the games themselves, I didn't like them from what I've seen of them. VI changed a bit too much for my taste and with VII, it was clearly visible that there weren't enough funds/time put into it. Also, both of them being boringly set in the "distant past" instead of progressing the story of V/Dark Messiah was another off-putting factor for me.

Heroes 7 works on Unreal Engine, so it's running fine on every PC that meets system requirements. It's a decent game, despite what some people say. It's spiritually closer to classic Heroes games than Heroes 6. You should give it a try. :)

It seems that CONFLUX doesn't work anymore. It always give the different error messages - from "wrong game key" to "cannot connect to the game server". So while the base content is still available now, the core gameplay feature of HOMM6 is not.

By the way, for some silly reason there is a copy of M&M X Deluxe Ed appeared in the ubisoft connect library. Feels like a mockery, to be honest. :)