Allen has always been inconsistent despite his improvements. He can look like arguably the best QB in the league some times, then below average other times. It's very rare to see a string of great games from him, and that's always been the case.
His decision making has always been questionable, he's not as accurate as other top guys. He has physical gifts but I don't think he always uses them to the best of advantage. A lot of factors I think contribute to inconsistency, but I think far and and away the reason is he is simply asked to do too much
Bills are hard to beat when they are winning in the trenches and guys like Singletary are giving you something. But if their run game (outside of Allen) is neutralized at all, they are extremely ordinary.
Defensively I think injuries have hindered them quite a bit, though I still think they have a good enough defense to make a run.
I think Bengals would have won the game Monday night based on how things were looking early as well as I think the Bengals are the slightly better team right now playing at home, but obviously impossible to tell.