My campaign in AC3 reset itself for no reason and I had to play that redundant pile of shit all over again to get back to the point I was previously at. It was a waste of time because little did I know that most of the side quests would be bland chores around the Homestead and the DLC was just awful. At least Unity looks like they are going to French architecture, which should be just as great as Italy was, versus how 3 was more environmental with boring two story cabins and the occasional copy/paste church. I never got to play 4; I heard it was an improvement, but I'm not into Pirates at all.

IV is far superior in many ways.

I wouldn't know, but I've heard it's on par with ACII.

It's buggier, but at least the story is slightly easier to follow, rather than just 'go here, kill these Templars'.