I just spend a bit more time playing around with the stealth and i think after cleaning out almost every encounter (except orcs & statuedemons) with one ranged guy (survivor) i think i figured out the problem.
if you keep out of range of their FOV it seems they arent even able to do a perception check on you - they just shutdown. They stop moving around or anything. I feel theres some tuning needed, like for example some harder dicerolls towards the stealthcheck or some mechanic that automatically reveals you after you attacked (which is the case, at least i think, if you melee someone), otherwise id had to think of the battlemechanics as broken. just pick two survivors, skill stealth and bow and kill every encounter without even been seen.

Thats it for now.