I've been testing Privateers.
1) The size and power of the privateer is dependent on the tech level at creation.
2) They can attack freighters, colony ships, constructors, and all enemy warships with no consequences.
3) Asteroid mines and starbases appear to be immune to attack from privateers. This is huge draw back to them.
4)Attacking with mixed fleets (privateers and your normal ships) will cause war declarations, whereas fleets of privateers can attack enemy fleets, etc with no consequences. I was able to kill most of the AI ships using 1 privateer fleet. There might need to be fleet limit of some sort.
5) The AI ignores privateers as far as I can tell (when at peace ), which means the AI has no defense against them late game.