My prisons tend to be fairly pleasant work camps. Nice decor, same food the colonists eat usually, mostly work farming and harvesting and stone cutting, also mining since I added a mod that adds quarries as a building to provide stone chunks, and cleaning since I added the lockable doors mod. It's just not very hard to treat them fine if in an easy biome. If space or food are more difficult to provide I just release all the ones I don't want to recruit urgently unless I have someone who's down with butchering them for supplies.
Current run I'm trying out the magic rim mod, wanted to check it out in combination with the medieval mod. Firstly, I'm pretty impressed by the mod, it seems pretty neat in general, the UI for it isn't bad and spellcasters are rare enough they don't seem too normal. But I may have found a way to exploit necromancy by shoving the necromancer in an ancient cryptosleep casket between reanimating batches of zombies. Normally they can't sustain more than 3 for long before they run out of mana and corpses start dusting, but by shoving them in the casket their mana isn't affected by their zombie horde, so it regenerates normally, meaning they can be popped out later and used to animate more zombies before heading back to the casket.
Currently sitting at 32 zombies and one living person trundling about, with 2 more live people in caskets one of which is the necro, so a tiny bit more than the 3 or so zombies per necromancer the mod intends.