I treat black monoliths like nuclear weapons and keep mine sealed at the bottom of my fort on the magma level in a closed and trapped room, and only bring it out in desperate situations. It will guarantee I win any fight it is involved in, but at the cost of zillions of undead and harm to my FPS. Using the "raise all dead on the map" spell from the necro alter is even worse. It obliterates any invader, but my FPS plummets.
I typically upgrade a few skeletons who have critical skills like weapon/armor smithing, mechanics, that sort of thing, but my skeleton militia squad typically ends up full and everyone gets a curious ritual knife and full leather everything.
Also, on entombing warlocks and skeletons, the idea is you shove them in a coffin before they rot. If they haven't fallen apart you can just get them back up, warlocks are cheap to raise, skeletons are literally 3 for a soul to rez. I just build a layer of coffins around my alter and ignore skeleton losses. After the fight I just chuck em all in coffins and get a warlock to spam "raise skellie". It starts to feel like a revolving door for my skellies they get in and out of their coffins so frequently...