Not sure I like at, considering the nerfing it does compared to powers, for instance I could take the Cloak of Shadows, a +2 to stealth for Ambushes stunt, and a +2 to stealth for maneuvers, given with ambushes you have the drop on someone, you can reliably set up to +6 shifts worth of power not counting the hypothetical character in questions Superb Stealth.
Or in other words a Legendary +3 for Ambushes. Then considering some gun stunts to compliment... well you get the idea.
I just don't feel its necesary over all.
By the Rules As Written, you lose some efficiency when you stack stunts. If you have two stunts that give +2 that apply, the final result is generally +3, not +4.
One guideline you could go by is the +4 you get to see through a shapechange deceit if the shapechanged object or creature does something unusual.
So a veiled mage who fires a fireball is going to give everybody a +4 to see through their veil. Maybe if the spell is significantly stealthier it would not have as great an effect.
I don't see a problem with a veiled character getting off an ambush on the first exchange. But maintaining stealth after firing should be MUCH harder.
And coordinating an ambush where people can't see each other could be tough. Hand signals where you can't see your OWN hands?
Don't forget it costs a shift for each extra sense you wish to affect.
I would certainly support a GM who ruled that attacking causes the person veiling to Immediately lose the veil. But stealth would still apply, and coming out of Veil if the target had had no inkling that you were there would yield a couple exploitable aspects to boost your effective stealth.
That means that Joe apprentice with no ranks in stealth is not going to be killer effective ambusher.