Honestly there is nothing that CSM won't do just the sama as Zerkers. They might be fearless...but at 6 pts more per model.
Have to take 2 units from CSM, BLs, or Zerkers for the slaughtercult so thats why i am fiddling with all three.
SInce they are good at fighting squishy targets, i'd say that cultists do that better. And no one cries when they die...