Been experimenting with this a bit as it seems to be a recurring problem with a lot of mods.

I don't think this problem is ever likely to happen when a city revolts to rebels because it draws units from the descr_rebel_factions list and if there are any errors in the record for the local rebels then the game crashes on load. The game also crashes on load if that record has zero units in it.

When a city revolts back to a previous owner the game doesn't crash if there are zero units available. The city garrison is empty afterwards, but that is all.

As mentioned previously, if there are no units buildable for that faction in the city then it draws units from the merc pool for that region.

One thing I did find is that in this case, factions can get units they don't have ownership of. I think this is the only time that can happen.

Unfortunately I haven't modded units/models/textures type stuff much so I don't know the type of errors you get but my guess is that it is one of those type of errors which doesn't normally show up because that faction can never normally use that unit.

Maybe?