Basically anything that requires a major change to one of their core systems. Unfortunately I have to be vague because these are commercial projects I've been involved in. I wish I could say more.
I guess you could say AA-AAA games? It's kind of hard to for me to know what those are these days ![]()
Let me put it this way. If you were trying to make a graphically high fidelity game that was something fairly safe that Unity was designed for like a single player third person game then you shouldn't have too many problems. Unity is just very rigid in it's program structure in a way that makes changing it potentially more difficult than writing something from scratch.