I don't think it's necessarily that they lowered their standards, but I think it's a little different when you try to take a formula that has been established to fit in timelines where combat consisted largely of melee weapons and up-close fighting and put it in modern times when, for all intents and purposes, enemies and guards and what have you should have guns and other things that can down a man wearing mere street clothes in one shot. And with architecture as it is right now, with buildings that are taller than they've ever been, many of which have relatively uninteresting and uniform facades, it's tougher to make a larger-scale environment in these modern scenarios.
As these aren't really the focus of the game, it would probably take way too much time, effort and disc-space to make convincing modern-day open-world areas that have a lot of people and guards who carry guns (or at least should). So they likely make them linear and directed missions with more scripting and less people overall because that's easier to pull off and be convincing.
These, of course, don't really apply to the Desmond sections of Assassin's Creed: Revelations, which were a weird departure and really could have provided a much more satisfying gameplay experience, but I was really going into those to delve more into his backstory, so I thought they were an okay enough way to convey that.
And I don't know if my experience is out of the norm, but the framerate can get pretty dodgy when I'm playing Connor, as well. Then again, good or consistent framerate has never been a strength of the Assassin's Creed series, so that's par for the course at this point.
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