We also don't know the damage scaling. Yes, Kreig and Zero do tons of damage to psychos and rats, but the Vex and Hive likely have defenses that only similar tech, such as Guardians with the light or Fallen with the Ether, can surpass. Keep in mind, Humanity was wiped out, and The Light is what allows the Guardians to survive in a wholly hostile place. The Vault Hunters might nor be able to scratch the Hive or the Vex.