Assassin's Creed's gameplay is a huge problem here. The series' combat is stupidly easy, and the sequels only made it even easier. It's bad when a franchise revolving around assassins makes stealth completely and utterly worthless. There's absolutely no incentive to play it sneaky when you can slaughter entire legions of guards without any effort whatsoever. There's no skill, no challenge, just a flick of the analog stick and a press of a button to start a congo line of instant murder.
It's beyond being a path of least resistance. "The path of no resistance whatsoever" is far more apt.
The story's also a complete mess. The Desmond stuff was fine as a framing device, a mere setup for actually interesting settings and characters. Unfortunately, it seems someone at Ubisoft made the horrible mistake of thinking that it should take more and more prominence with each passing game. (Game Dev Pro-Tip: If your main character's distinguishing feature is that they are voiced by Nolan North, go back to the drawing board and try again.) If the writer's intended for that from the get-go, then they executed the transition in the worst manner possible.
It's a shame, too, because the games made use of some great, unique settings (places and time periods normally ignored by most other games) only to trip over itself to deliver an inane, uninteresting "end of the world" plotline. Just a whole lot of wasted effort all around.
Ubisoft kind of drove the series into the ground and wore my interest in it down. Still, I'd like mind if the developers gave the series a little breather and returned to it once they had some new ideas for the combat and storyline. I think there's still potential for great things in the franchise, but it's just being squandered as of now. This won't happen, of course, but a man can dream, damn it.