My understand is that it desperately needs the fan patch to be remotely playable, but it's otherwise probably the best adaptation of 3.5 D&D in a computer game. Storywise though we're well on the Icewind Dale side of the Icewind Dale-to-Planescape Torment scale
No, I think the built-in setting thing is a thing. Even at the height of Fate's popularity, you weren't hearing about Fate Core as much as you were hearing about some of the licensed games based off of it, like Dresden Files or Leverage.
I think that going modular with "class"/career design, ala D20 Modern or even Star Wars Saga Edition, could potentially work well. Going the traditional 5e class-and-archetype route where multiclassing is an afterthought or charop thing would not be a very good fit.