Yeah, AI needs a lot of work. I don't think it'll be sorted in Civ 6 but I hope they really look at it for Civ 7. It's the way they handle difficulty level increases that causes most of the issues, as their AI is not bad on some difficulties, but for higher difficulties they just give extra stuff/cheats to the computer, and lower difficulties they forget their objectives too much and are too passive. It skews their priorities and everything gets messed up - pretty much why domination victories take over at Immortal/Deity, and Civ tier rankings all are domination based for the most part. It also causes your first 50-100 moves to be nearly identical as you're playing catch up.
Civ 6 is still a really good game at this point though; especially with the latest expansion.