Claws is one of the few Supernatural Powers based off the Mortal Stunt rules, namely:
Give a +2 to an attack’s result, applied only if the attack was successful, under particular conditions
. May have a lesser benefit in a secondary circumstance.
- Example: Lethal Weapon - Your hands are lethal weapons. When using Fists to strike an unarmored opponent, you are considered to have Weapon:2. Against opponents with Armor:1, your Fists attacks are Weapon:1. There is no benefit against more heavily armored opponents.
It actually seems slightly better than the stunt and has the same refresh cost.
As for hiding them: Human Guise is a [-0] power, and I'd say that would make them retractable or similar. (Appear human until claws pop out of your fingers, and might have to make rolls to keep them retracted when angry as per Human Guise, p176.)
The advantage of the power over an item is that the power can't be taken from you short of serious mutilation.