In my experience the zealot’s grenade could stun everything until now. Yesterday and today I had multiple instances when my grenade failed to stun muties, berserkers, crushers. And if I remember correctly, they sometimes get stunned but sometimes not.
I’m fairly sure it’s related to damage and/or stagger they’ve taken before the stun goes off. If a crusher’s fresh, I don’t think it affects them, but if you gave them some fisticuffs before throwing it should stun them. At least that’s what I’ve found, who knows how it was designed…