Phantasmal Rogue will try to get behind a target when possible, and it hurts quite a bit (its backstab is designed around hitting from behind). It also gains the +damage and +fury traits. Just, like most illusions, it doesn't have a lot of health. I'm pretty sure it has access to the +health trait, but not Signet of Illusions because you're downed.
But that still doesn't prevent it from dying when focused by one major attack from pretty much anything. And if it can't be behind a target, does about as much as a warlock when the only debuffs are your own (i.e. can't kill anything by itself to hope to save you).