Nemesis Hammerhand requires a half action to use, eliminating the possibility of using it and making more than one attack and also moving on the same turn to attack.
So the only time it can be used is if you are already standing next to what ever you are swinging at.
Grey Knights can get Swift Attack at rank 1.
So the requirements for making multiple attacks will always be met when using Hammerhand would be valid. But why would anyone ever use Hammerhand?
When would it possibly be useful to use a psychic power to gain 4-5 damage when one could just make a second attack that will do 20 damage?