Being zombie will never be fun, regardless of what you do. That's the nature of the game, and there really isn't anything that can be done other than making being human abysmally unfun as well.
Zombies already have innate damage resistance, as well as significantly higher health, base damage, and attack speed. I've seen quite a few good zombie matches where two grenade launchers and a sea of t5 weapons couldn't stop the horde. The best thing to do here is buff the damage resistance of low-pop zombies, not necessarily nerf a specific weapon.
The only nerf I stand wholeheartedly behind is reducing or eliminating the grenade launcher's knockback. The knockback is cancer. The knockback is OP. The knockback needs to be removed.