It isn't only about 1UPT causing problems for the AI, it's not as simple as that. Yes, programming an AI that can play well in a 1UPT environment is far more challenging than one that only has to deal with hexes, but the sad truth about Civ V in general and its AI in particular is that it is simply a half-assed effort. That really is the underlying theme with V: it's sloppily done. Firaxis has never treated it as a game they were genuinely enthusiastic about and interested in perfecting - it just had to be good enough that it would sell, and then sell some more with pricey expansion packs. I'm not saying V is a bad game by any means - if I thought so, I certainly wouldn't have spent so many hours playing it - but I can still tell perfectly well that it it remains a game of half solutions and just doesn't reach the heights of refinement that IV set out for. V works because the base premise of the game is so enticing - because a turn-based 4X is fun. It works because of that, not because V is a strong, well-built game in itself.
It's not that IV was perfect - it certainly had its kinks, as well - but the difference is very noticeable. IV feels polished, and you can tell there was much developer love cast unto that project. It carried more than the purely commercial interest V does. IV is like chess: a classic, timeless game. V is a game to be sold to the masses, then discarded when something better comes along.