Vertibirds dropping like flies seems very realistic given their exposure. I mean, as machine gun and rocket platforms, you'd expect them to be treated as airborne artillery; Keep them on the ground until a spotter calls in a target, fly them in to a deployment zone miles away from the action, fire off some bracketing rounds with feedback from spotter, and then rain depleted uranium and fire down on whoever the spotter has picked out.
Just doing that is dangerous enough but flying VTOLs into direct confrontation is a good way to lose them and their crews. And, competent crews, by the way, are much harder to replace than aircraft.
So, for me, the rain of vertibirds makes sense given the BoS no longer have competent crews (having been at war with all sorts for two centuries) and, well, if those BoS folk have been gathering up tech, I would be very surprised if they did not have steel mills, precision machine tool shops and manufacturing plants capable of churning out vertibirds like there was no tomorrow - especially with all the building scrap around. Just smelt those I-beams, engine blocks and old metal sheets, drop forge the machine parts and mill that sheet-steel. It's not like there's a price of doing business here. Good crews, on the other hand, can't be churned out like that and, well, the way those vertibirds are being flown is as if they're being crewed by whoever the BoS can grab and strap into the cockpits...