As written, getting two successes on an attack roll, which is entirely possible if not easy by Pushing a roll with with high Fighting or Agility, allows you to use the Double Damage stunt to straight double the damage on a hit.
The problem is when you combine high base Slugfest, high Strike, and whatever other damage boosts you get ... and double *everything*, you can be in a situation where you can basically one shot even very tanky opponents since there's no way to double Protection or armor to defend against such a thing. Now obviously, not everyone will be this strong, but if the group is ever fighting something traditionally very strong like your average Viltrumite doing 5 to 7 base Slugfest damage every hit (I consider big shots like Omni-man and Conquest to be outliers, but god forbid PCs run into them), that's a 10 to 14 damage hit. If a character is on the edge of being broken, that's pretty much an automatic lethal hit.
Even more concerning is the Deadly Strike/Blast boost that allow you to land hard crits on two successes as it basically allows even high HP characters to take crippling or even lethal injuries each hit since crits injuries *always* scale up from previous ones.
Now perhaps it's not that big a concern since, in principle, people should try and Block/Dodge attacks, but there's plenty of situations where that's not possible, for example if you move and attack on your turn, leaving no actions left to defend before your next turn. Maybe it's that way by design?
I haven't had a chance to playtest with my group, but perhaps other folks can chime in on their own experiences here? As a GM, I want my players to stay alive to fight another day ... but at the same time, if my players are constantly double damaging or auto-critting every attack, I almost feel obligated to have Big Bads to do the same ... and then feel like ass because, woops, dead PC.
Moderator Action: Moved this topic to a more appropriate place (it was in Coriolis: TGD).