My major concern with not being able to reanimate a corpse more than once is that a player who invests in Necromancy has no other or very limited alternative abilities. That leaves the build walking a very fine line between being too powerful (minions don't die) and nearly useless (minions die leaving you with nothing).
If they can be reanimated repeatedly but you're vulnerable while doing so then it will be easier to achieve balance since you can allow for them to be less competent in combat and work with the idea that they should be cut down in a fight rather than attempt to walk the line between keeping them alive without overpowering the build.
There's already going to be locational wounds that inhibit the performance of a character. That's a great opportunity to balance Necromancy in a realistic and immersive way by accumulating these locational wounds with each reanimation so the minion becomes noticeably and visibly more worthless with reuse. At the same time it helps get round the problem of balancing between minions being too strong to die or dying too easily leaving the Necromancer with nothing.
What if a minion could be reanimated repeatedly in a progressively damaged state but after accumulating too much damage would burn up? In this scenario equipment degrades normally (or possibly slightly faster) but takes a severe hit to its condition, possibly breaking, only when the minion actually burns up. The trigger to cause a minion to burn up may be sustaining multiple locational wounds to the same area, making it unpredictable but increasingly likely with repeated reanimation.
You actually have a very good point, necromancers would mostly just be relying on their undead minions. Although, wouldn't the type of thaumaturgy used to reanimate a corpse also have some other spells the player can use to do damage with bones or whatever, a la diablo II necromancer?
Thinking on it, maybe having undead minions level up skills and such would be a silly idea. I guess I just thought it'd be neat to just randomly have a skeleton hanging out as a normal party member.
Perhaps a better way to go about it would be to have newly resurrected corpses be weak, sluggish, and pretty dumb, but, over time (the amount of time is debatable, but I'd maybe say like, half an in-game day to a few days, or something?) they'd slowly get stronger, swifter and more able to make their own decisions. So, at first they just stumble after you blindly since you're providing their life energy, and you have to give them direct orders to attack things or control them directly like a puppet, and they'd be fairly weak, but able to swing a small sword sluggishly at enemies and will be taking a lot of hits, and eventually they will guard you intelligently, and be able to equip better weapons (but only as strong as whatever they were like alive, maybe?), and use footwork and more precise strikes to bring enemies down.
I also rescind my statement about just instant perma-death for minions, what's the point in resurrecting undead if they can't take a few hits? I like the thought of having them be affected by wounds and such until they're useless.
I'm not sure about having a big long ritual in a prepared area to revive something. Maybe only have it necessary for very large creatures? It's pretty cool, but it'd be impossible in the middle of a fight as you don't have stuff ready.
I literally just had an idea whilst typing that. Have the ritual resurrection an optional thing, but make the continuous focus drain reduced or halved when it's used, but still have it possible to revive undead in a fight . That way, there'd be a reason to do a ritual and the player could still be able to revive undead in a fight if needed.
Also, yes please on terrified villagers attacking minions, though maybe make it possible to reason with NPCs and explain that your little pet skeleton is harmless, or something? Maybe if you deck your undead out in clothes or armor so it's undead-ness isn't really visible, the villagers don't react more than just being creeped out by the strange person?
Have we all agreed on having undead minions present simply taking a chunk out of your max focus pool, depending on how many undead and how strong the original living being was you're wanting revived? If so, good.
I can't really think of anything else on the topic to cover, really. We've pretty much covered all the bases.