I have to agree that it seems that features would only impact the the amount of dice for the person possessing those features. For the zombie the "horrifying" and "unrelenting" features would add positive dice to the zombie's actions. The same way if a PC had a "body builder" feature wouldn't add negative dice to the zombies attack.
Actually if the zombie were grappling with a PC with "bodybuilder" I would be inclined to give the zombie a negative die for that. The extra strength would make it easier for the PC to hold the zombie at bay, thus making it harder for the zombie to make a successful attack.
Bodybuilder would not help if the PC were the target of a rifle or pistol shot though.
There is a reason most RPG don't go that route... I could just as easily grab a bodybuilder as I could a guitarist.
He would free himself a lot easier though... but that would be on his turn.