8th gate isn't like kaioken or supersaiyan where you just ttransform and youre strong. it depends on your bodies physical strength and condition. might dai couldn't even kill all the 7 swordsman if he did it. if neiji did it he would be weaker because gates uses hard first style while neiji uses the opposite.
He probably would've been able to rotate fast enough to block the Juubi's wooden spikes, meaning he probably wouldn't have died. Screw powerful, he'd be alive.