Single player most of the problems people had were in the realm of broken promises. (IE: Not redoing the story to reflect retconed lore, not redoing all in game cinematics to be like the strath one, completely removing the custom campaign feature, stuff like that).
You can play through the original campaigns without issue though, they work. It just leaves a really bad taste because of everything they promised to do and simply never did.
Re-reforged is a great mod too but its not done yet (I think just the human campaign is? I dunno)