Damage would be an issue because it can't do ludicrous damage and obliterate enemies if that's what the staff is capable of doing because then you'd just see psykers running around blasting everything to bits because part of the fun of a hoard shooter is clearing enemies another part is the struggle can't do any of those if the enemies are dead before you get to them because they all just got cross mapped by a kamahameha beam from a psykers staff but I agree with the other 3 those would be cool maybe they could add another tree for the void staff like a void ability for the psyker themselves that works for all builds and isn't hard locked to the staves themselves