I’ve encountered a fair number of both brainless and listeners. They both pretty much just seem like target dummies for very new survivors to practice combat skills on safely, if you are lucky enough to encounter them on day 1-2. After that they’re pretty much just trash that poses a threat only inasmuch as they can body block for bigger zeds or exhaust your stamina in an extended fight.
I kind of expected the Listener zombies to be very noise sensitive and home in on shouts and combat sounds from long distances - but I really haven’t noticed any such behavior from them? They seem substantially less responsive than a basic zed, even where listen checks are concerned. Dunno if that’s a bug or what.
Have not yet seen a skull zombie as of day 60 or so of a 0.D playthrough.