Correct me if I'm wrong but I did not see anywhere in RAW mechanics to handle attacking unaware foes.
We ruled it that the target cannot defend and thus automatically receives a critical strike if the attack is successful but the results, at least at low level seem pretty underwhelming. At worst it can result in 2 critical strikes, that add up to either a severe or permanent injury - but you're left with the victim still standing, at full endurance and pretty pissed at you.
Did I miss anything?