All those "broken characters" in other series have believable, reasonable origins for their powers - in the context of their universe. Saitama doesnīt. His powerīs origin is something everybody of us could do, and something that millions of people actually do. Without the same result. Thatīs what makes his power absolutely unattainable.

With Goku, you always have stronger enemies because "aliens!" is a valid explanation within the universeīs context.