|Here we see the|
beast in its natural habitat
Sniper drones can lay out a devastating amount of firepower with their BS5, rapid-firing, 48" range sniper rifles - pulse weapons, so they benefit from an ethereal's Storm of Fire invocation. This means that when your target is within 24", each sniper drone can fire three shots.
Plus, sniper drones have stealth. And they're T4. And they can move and fire, then make a 2d6" thrust move when they're done. All this for just 15 points a model - just 148 for a full unit of nine drones and their accompanying firesight marksman.
If you're not sure how that translates into damage output, consider this: they can drop any greater daemon in one round of fire, with just one unit of drones. 27 shots, 24 hits, 12 wounds, oh look that Great Unclean One is dead. No problem.
Iron Arm making you T9 this round? Too bad, these tiny murder machines don't care. Even against armored targets, 1/3 of your wounding hits are going to rend - that hive tyrant's not gonna last much longer than the greater daemon.
The only thing they have trouble with is vehicles. But that's what the rest of your army is for, right?