Fics which portray empires as "rape harem" tend to forget that the captured nations might have lost some power from their annexation but they still have at least nearly as much military training and centuries of experience as their conqueror does. I'm picturing it more as the empire learning in a hurry why it's a bad idea to bring a bunch of semi-invulnerable people who want you dead into your home. Even if they're compelled via orders from the human leader not to try to kill their master, there are plenty of ways they can make his/her life just as miserable as theirs is. Especially if the nations in question are siblings and already know how to push each other's buttons. Within a week the empire's begging their leaders to send the other nations back.