People just didn't like AC3 because there was more exploring in the wild. To me AC3 had the best amount of DLC.
AC4 was a great game, but a joke with the "Season Pass" you only really got Freedom Cry and the rest was silly cosmetic stuff.
AC:Unity is alright so far, I haven't really played it much, because everything has came out at the same time.
People hated AC3 because it had so many bugs, quite possibly the most of any AC game in recent memory.
The gameplay was fairly cookie cutter with the annoying bit having to travel such long distances to do everything.