Eh, there's still no reason at all to have them NPC only. They're not mindless, or significantly more resilient than forces available to other factions. (Reanimation protocols are more common but less reliable than most Invulnerable saves. Call that a draw.) Their weapons don't make for harder hitting squads than are available to other forces (standard weapons can glance anything if they get lucky, but they have to ride that luck since they don't get the weapon options of other factions). On top of that, they're slow as get-out and crumple like paper in close combat (a few dedicated edge cases aside). They don't need to 'butcher the lore' to make them semi-balanced with everyone else. They already are.