I think the fall back will be useful on alot of slaanesh stuff, Blissbard archers will likely want to be at max range from threats so might be able to keep a little more distance, or stand and shoot for extra few depravity on the chargers before being killed.
The +1 save on myrmidesh might help them work well as an anvil, 3+ re rolling with possibly negatives from glutos/fiends/shardspeaker and mystic shield, if we see a meta shift to some grindier 2-3 turn combats the tankier painbringers might also help the shardspeaker be close enough to use its abilities.
As for opponents, the fall back and stand and shoot might not hurt key units we have are so fast anyway its likely not going to be an issue (sigvald should be averaging between 9-12" seekers can run fast enough to just start their charge close). Stand and shoot might actually help us get more depravity spread around if all ranged units start plinking us as we go in. Falling back with keeper or other daemons could be neat as well, if we fall back and they still barely get charge off they are likely in a very spread out spot making locus scary if it triggers.
While the caps will hurt glutos+fiends builds, what we are attacking into also wont be as crazy buffed up.
On the twins and other stuff, I would actually like to see them get a new depravity mechanic, let them spend depravity in smaller chunks to hand out buffs to units around them, with the idea of them channelling the depravity to drive worshipers further, Would be useful for small amounts of depravity here and there, and would make the built in extra points cost on units not feel as bad if you can super charge them. Also works thematically as a twisted version of aether quartz from lumineth.