IF Grimghasts got nerfed how would you want to change them? Their role is...strange. If their points go up, they become useless as they bump into other units of similar cost or knock out those units of similar cost. Their points wont go down. They wont change the warscroll cuz that would just ****** off everyone. I think the real issue is that Nighthaunt itself is ****** compared to GHoN. Their spells/items/allegiance in general is sub-par.
The only thing I can think of from a balancing point of view is swap Grimghasts to the allies spot. Give points reductions to other Nighthaunt units and shift around other Nighthaunt units from allies to useable in the Legions.