In terms of ubisoft working on the same project every year, I would argue that they should only have bugs if they are introducing something new. If they are reusing assets from previous installments, I would expect them to have little problems with those assets because they have been tested and polished in their previous series.
The main question is, did the new development environment of the next generation influence this? New environment means adding a steep in the learning curve.
Also, from what I am hearing the bugs are not even that critical(I could be wrong)
I'm pretty sure they rebuild a great deal of the assets--or, over the past few games they have. Take a look at how different the architecture in the games has been. Though I'm not sure what you mean by assets--when I use the word, I'm referring typically to 3D models. Do you mean the scripting, AI? if so, I think from Revelations on has had a tremendous spread of different types of things--Revelations as the bastion of the Ezio era, then III with the "500 NPCs at once" and the semi-revamped parkour/combat, tree climbing, hunting, then IV with the ships and weather and the sea, and now Unity with the pillars revamped and (I suspect) pretty different AI tech. I do have a further statement to make.
Pity, I thought they learned how to do endings with Black Flag.
That right there was a damned good ending tbh.
YUS.
But it must be remembered that the AC games are made by completely different directors (Amancio's last game was Revelations), so this has a huge effect on the way the game turns out, both in assets (talking to you, Hannibal) and in story (Snook). I was actually just reading about Final Fantasy, a series that does something much similar.