The assassins are very powerful "diplomats". Even the newly minted assassins have at least 5% chance to kill any person. So a squad of about 10 assassins can kill anyone in 1-2 turns at most. Excommunicated? Send the assassins. The Pope is a PITA - send the assassins. Playing as an orthodox nation and going to kill the last general & king of a catholic nation? Send the assassins first and kill them before attacking.
But not sure what to do with the foreign princesses - do the AI try to make alliances or try to steal the human generals?