It's certainly new and allows for weird combat situations, but maybe the wandering dead affect the FPS too much after you get several hundreds of them.
It would be far, far easier to program though.
I'll be honest, that change would pretty much kill the mode for me. One of the things I honestly like about Warlocks currently is being able to control my population via raising zombies or skeletons. If I wanted mass produced "pets" I'd play gnome and build clockworks. Has the additional perks of being able to pasture them strategically and not dealing with massive amounts of pathfinding.
Not to mention that playing Necromancers that are able to produce nothing but mindless undead really feels lacking from a thematic point of view.
"I'm a powerful Necromancer! Bow in terror!"
"Oh, you can raise the dead to do your bidding?"
"Well, I can raise the dead yes."
"They don't do what you tell them?"
"Well, no."
"What do they do then?"
"Well, they roam around mindlessly and kill anything they find that's not me or my fellow necromancers."
"Ahh, must be a pain to keep livestock around."
"Living pets as well. First time I raised some dead, they killed my pet rat, 'Plaguebearer.'"
"Oh, that blows."
"Eh, I raised him next."
"Oh, well that's good I guess. See him still?"
"Well parts of him. He ran off and tried to kill a moose. Smashed it to bits. Raised those back too."
Here's some of what's come to mind for how I'd think of updating Warlocks/Necros:
Zombies: Left more or less as-is. Cheap, mass-produced unskilled workers. Perhaps give them a limited shelf-life of a year or so to compensate for not having to feed or give them rooms.
Skeletons: Skeletons would be meant for military, labor and middling crafting. Construction would be similar to what it is now: bones, totem, thread/leather, and a soul. Skills could only be upgraded via the inscription process(which I'd probably increase the cost of) but there'd be a cap on production skills due to skeletons lacking the "creative spark" that'd allow them to produce high-quality goods.
Ghouls: This is a group I'd change a bit since, personally, I don't see much of a point to having them at the moment. They're restricted on weaponry, production skills and require food/lodging. What I'd do is make them an improvement over zombies: Faster, able to naturally learn skills(at a reduced rate) with no cap, but require food/lodging/clothing.
For creating them, I'd use the "prepared parts" concept over directly turning a prisoner into a ghoul:
Two arms, two legs, torso, head, soul and heart create a basic ghoul with the additional options of constructing two head/four arm/both variants then or add the parts to a already created ghoul later.
Additionally, perhaps have an inscription-esque upgrade workshop that uses prepared parts and souls for them to increase attributes or gain a temporary boost to skill learning(By group to reduce reaction clutter/micromanaging). Arms could be used for strength/endurance, legs for agility/endurance, torsos for toughness, heads for skill boosts and perhaps have hearts usable for a recuperation boost. To help balance, give them a high rate of skill rust(on production skills at least, perhaps labor as well) to keep any single ghoul from being an undead jack of all trades.
I'd also add a cheapish zombie/skeleton vermin hunter option. Makes little sense to me to have to spend souls to "buy" vermin hunters when the necros can make fully functioning zombies/skeles/ghouls.
For replenishing Necro ranks, I'd see about making it possible to "tempt prisoners promises of forbidden knowledge" to make apprentices. This would be a pass/fail mechanic per prisoner kinda like how strip-searching is. Either they are willing or are not and most will not be willing and some may even take the chance to attack. Apprentices would be capable of learning the same skills a full-fledged Necro can at a reduced speed. After a period of time(year or more), they become full-fledged Necros.
Sacrificial rituals: Instead of using Necros, let it use prisoners with a chance to fail. However, for a vastly improved chance of success, those fledgling apprentices can be used. Their tainted souls make far better catalysts and honestly, why take apprentices if not to use them to do things you don't want to do but can't use your undead for?
Like I said earlier, insane, planning on trying to figure this out myself, etc...