This whole Screed Unity thing is very confusing. From what I've gathered AC:U uses their in house engine, BUT there is a different (handheld?) game in the AC series that did use the Unity Engine.
I also think it's kind of important to remember that if you go by the forums it's going to seem like a large number of people are having issues because beyond the regulars and hardcore fans, the majority of people who hunt down a forum for a game are people who are having issues with said game. So even though a relatively high number of people are raising their voices about issues, they might make up a smaller percentage of people playing the game than one might think. It's really hard to say without all of the data.
RTG probably has a better idea how many people have bought the game vs. how many people have had serious issues.
Not to say I'm not with you, Lee. Immediately post launch, I was definitely worried because it seemed like everyone was having issues from my perspective over here. But I also haven't followed that many game releases as closely as I've followed this one, so I wasn't entirely sure what to expect. Things got sorted quickly and RTG was great and now that I have a bit more perspective and information I feel much less concerned than I did in the thick of things.