Civilization has never been a bastion of great computer opponent strategy, but it typically was good enough to suspend disbelief and enjoy. However, VI was just horrific. I played a lot of VI and stuck with it for a few years but the AI still would declare war... not attack, put little to no effort into defense.
It was a fun game and I got my value out of it but eventually it just became frustrating with the AI sort of rage quits and almost behaves like a human greifer just taking random actions that make no sense at all / kill the fun of the game.
I've played civ games constantly since civ but unlike the previous games I just had to quit VI because the AI made it seem so pointless when it ruins a perfectly good game by just going bonkers after you've put hours into it...
Unless it has gotten better, if you have a low tolerance for wonky AI, you might want to pass on this one.