The vex army is infinite and exist in multiple universes. They work in close relationship with time, sometimes even managing to travel through it (Atheon) The hive not only have seen many worlds fall to the darkness but exist in alternate universes with a different God ruling each one. (Crota's was the most accessible so it's fair to assume that a god immune to bullets or any sort of damage except a sword in his own chamber is the WEAKEST). That also means many more universes with many more powerful gods. The cabal are the strongest military force in the destiny universe. They have conquered more worlds that Earth has even recorded. "They blow up planets and moons just for getting in their way" Not only this but guardians are practically unkillable, they can be revived constantly by other guardians. The abilities of guardians make them very hard to kill.