95% chance to hit on each of two attacks. Makes a chance of 0.95 times 0.95 chance of hitting with both attacks.
Take a nova round of a slayer for example. (I know bad example as it is also from HotFL)
You can easily achieve a hit bonus of +9 at first level. Damage is about 1d10+8. Against a first level brute you deal 3d10+16 damage with your nova. Your chance to hit with a little help from a friend it is about 95% hit chance against AC 15 IF you go for poised assault.
Compare this to a thief´s 4d6+14 (18-38) damage on a nova. Even a striker with about no nova potential comes ahead of a thief.
The Thief I just put together:
DPR Ranged Dagger (no Sneak Attack): 0.20*0 + 0.75*10.5 + 0.05*12 = 8.475
DPR Charge Backstab Acrobat's Trick: 0.5*0 + 0.95*29.5 + 0.05*44 = 30.225
38.7 DPR
And because he is using Backstab, he continues to hit higher AC foes when Nova is needed. The Slayer's DPR drops faster against the type of foes that a Nova is needed against.
Assuming that the Slayer you just posted can get to 95% (which is not guaranteed: +3 prof, +5 str, +1 talent, +1 poise = 80%):
DPR Power Strike: 0.05*0 + 0.95*19 + 0.05*28 = 19.45
DPR: 0.05*0 + 0.95*13.5 + 0.05*18 = 13.725
DPR: 33.175
And with +8 damage, it means that he cannot be Human. So, no second feat.
He didn't wipe the floor over the Thief with his nova. He actually lost. And that's with help to get him to a 95% hit range (normally, he would be at 75%, 80% with Poised Assault, 85% with Poised Assault and a Charge).
The Thief does not need allies giving him an edge. He gives it to himself.
No, the Thief is not the best Nova-ing Striker. But, he's not terrible and he can hold his own. But, day in and day out, he's hitting, especially against higher AC foes.