Have not played Civ VI. I own the vanilla version because of Epic Games' free game every week thing. I'm not so happy about de-stacking units, cos it makes the military aspect too slow and involved. And apparently the AI can't handle it.
That's one of the great things about Civ IV; the AI is slightly competent. They still cheat at higher levels, but it's my understanding that going from IV to V or VI, you can basically move up at least one difficulty level.