Despite often being billed as a stealth series, Assassin's Creed has always shied away from allowing its players to actually play stealthily. Hell, it took eight games before the franchise even added a crouch button, so it's no surprise that the developers haven't promoted sneaking around and attacking from the shadows as a viable playstyle.
Over the past few releases, the series has admittedly attempted to make its stealth options more robust, but specific problems still make it one of the worst systems of its kind. Enemy A.I in particular makes staying hidden a chore, as they're either too dumb to notice you from an inch away or so eagle-eyed that they spot you behind cover.
The maps pose the biggest issues however, as they're rarely actually designed for stealthy play. Sure, you can hide in conveniently placed bushes or attack enemies from above, but for the most part players have to fight with the game and use exploits if they want a true stealth experience.