While they're trying to pry parts off of him, he's punching them to pulp or tearing their bodies apart with his claws. Having six limbs doesn't hurt either, and it'd actually hurt either of them to try and strike Grievous' frame ala Kenobi in RotS.
Also not bought on Maul indisputably being more skilled than Grievous in terms of unarmed, but either way Grievous' unarmed feats are just better.