Yeah stealth is weird often, zombies really wake up only if you are in thir aggro radius but once you enter POI when you shoot one with rifle, you are always in the aggro radius of several zombies and you wake up them. Stealth skill work fine for me, if you combine it only with cloth armor, and bow you can stay almost next to them + bow has 200% steath bonus combine with assassin skill you can kill almost every zombie with just one arrow to the head even on higher difficuilties like warrior or survivalist.
Weird is when you stand outside of POI, there is some parking with some sleepers you can just shot them from one meter with shotgun and you don't wake up anything because you are out that aggro radius... But what break the stealth most for me are skripts. Often in the POI are rooms with skripts, once you enter them every zombie wake up no matter how good is your stealth. I hate that.