First of all, assassins creed black flag has no bugs. Assassins creed unity had bugs since the day it came out, also frame rate issues and many glitches, making the game not very enjoyable. Assassins Creed black flag really doesn't have any of these problems, making it very enjoyable with hardly any problems.
Assassins creed black flag had a more interesting plot line and Edward is the third best protagonist I have seen in a assassins creed game, Arno being the second worst, he just seemed so bland. Moving on in Unity they added the character progression system was also flawed being as you had buy using points (cant remember exactly what the points where called) which is fine on the face of it but these ability where ones that let you do things which are easy to do anyway e.G stab someone when they are on the ground. Apart from that they stripped half the features out of the game and called it "streamlining" and the few new features that where instituted was frustrating or annoying to use, none of this was present in black flag in fact in black flag they expanded the ship mechanic and added the boarding of other ships. In unity the free running was even worse than in black flag. Also Unity had massive lag spikes on console, and I hear that it did not work very well on pc either. The only redeeming thing about unity was it graphics which where gorgeous (even if they caused the loading screens to take for ever).
Personally, I haven't had many problems glitch-wise as most people seem to have. Sure there's the annual guy who jumps onto a crate, spins a bit, then goes off on his usual stroll, but nothing game breaking that would make me like this game any less. I like Unity better because there's a larger amount of buildings to climb and more ground to cover. In the words of IGN "7/10 Too much water", I wasn't very fond of sailing, though I had some fun with it at times, it got rather tedious.