Prometheans are boss battles in themselves, to be honest. They call in as many Crawlers as they want to distract you and pressure you, they call in Watchers to counter any of your tactics at getting close/doing damage to them, and when shit hits the fan, they charge you like crazy.
It's also fun to have a Promethean distracted by your teammates and you think "sweet, I'll just sneak up behind him and get an assassination" only to have the Promethean smack you in the face with his elbow going "NOPE" as you go flying 20 feet across the play space.