Ash is balanced. You know why? Because letss compare to Poison for a sec
Even thought poison has better overall multiplier, ash has more impact damage. So in the long run, it will in fact do more damage than poison if you have enough power. And in terms of synergies it’s really good too.
The key factor that contributes to its greatness is the clouds. You almost always get a few ticks of damage per attack, which is like 50 extra damage and crowd control.