How common are empires in fantasy that are oppressive or unjust (ETA:) and whose oppressions are a plot concern, but are not run by Evil Deities etc. and do not exist to be the opposition for the protagonist's polity? I'm thinking of David Anthony Durham's Acacia trilogy, a book that's not out yet so I'd prefer to avoid discussing it in case it's a spoiler, and . . . ?
I suspect, not very, as fantasy is well-known for its aristocratic preferences, but I thought I should ask.