Grav weapons aren't nearly as strong as you are making them out to be.
They are roughly a S3 weapon for all intents and purposes.
- wounds S1 targets on a 2+. (same as a S3 weapon vs T1)
- wounds S2 targets on a 3+. (same as a S3 weapon vs T2)
- wounds S3 targets on a 4+. (same as a S3 weapon vs T3)
- wounds S4 targets on a 5+. (same as a S3 weapon vs T4)
- wounds S5+ targets on a 6. (worse than a S3 weapon vs T5, and equal to a S3 weapon vs T6)
Unlike most weapons in this game, flesh wounds do not make it easier for them to inflict damage.
They only get better late game after people have taken injuries that have reduced their strength.