I think for pets one of the problems is threat generation. So far I've experienced this with my locks. They will hold for a bit but when dps picks up then the boss will break off, go after you and 1 shot you The voids have enough health just not enough threat.

At the moment I'm mixing my group with prot war and the mobs stay glued onto him. Shield bash/revenge/sunder generate more continues threat, than the taunts from the voids.