We have a necromancer in one game and the DM does try to target the skeletons. Tries to. We typically one round almost every "boss" encounter between their sheer numbers, a Gloom Stalker and a Pally.
We lose skellies now and again but have found that they're pretty damn good uses of spell slots over time with near infinite value. The only things that threaten them are hordes of enemies. Everything else he does seems to be cantrips, reaction spells and wands so he's really playing the spell slot value game. And our real constraint is space; in a 10ft corridor they're gonna be shooting through allies therefore giving enemies free cover, and that's even assuming they can all fit.