I think he's good, I play him like cage. Land my b22/f21/d1d2 xxLKick for free pressure and its like 20% so in theory you only need to guess right 3 times to take the game. But that's just how I play
He is godlike until people know the matchup. In 3 months Flash will truly be bottom. JC does huge chip damages, he jails, and gets meter doing his pressure on block. Flash does none of those 3 things (and he is more meter dependant than JC).
The f2, 1, whatever is duckable which means that they can punish you for doing it. All the other strings are just useless against crouching opponents except d1d2, because it's +2 and low but it has a shit range (fortunetly because it would be overpowered if it wasn't the case).
Also, when people will learn they can d2 against sonic pound on reaction to punish it, this option won't be that good anymore.
His d3 is punishable on block (flash is the only one).
b2 is the only decent whiff punisher because of his range, but it's too slow to be viable. LK is -5 on block so it's safe but you gotta back dash everytime you do so.
So yes, the way you play the flash is great because no one knows him. But sooner or later, it will be harder to get them on hit and so harder to win a match since there is no damage/meter hitting /w normals on block in that game.