Same as with the BSF Traitor Guard, I 'd say. One box of them (1 Renegade Psyker, 2 Negavolt Cultists, 2 Beastmen, 7 Guard) would probably make a good base for a Killteam. The Servants of the Abyss (8 Cultists, 1 Firebrand) might be trickier to adapt.
Funny that GW killed off those boxes, I...