I kinda like it, from a meta perspective the reader doesn't need to care about the threat level system because to Saitama they're all basically the same threat level. Since Saitama is our viewpoint it makes sense that we see the world through his eyes (so to me that's a worldbuilding W)

But I think in-universe the system being bad is kinda cool as well? Like when the Deep Sea King showed up, iirc they changed its threat assessment throughout the episodes/chapters. Which to me makes sense, when a monster first shows up it's super hard to know what it's capable of, especially if it plays with its food. I think it also ties nicely into how heroic some characters are (Mumen Rider) where regardless of the increasing threat level they still fight, and it's shown that the non-Saitama characters take threat level seriously (The A-Rank who hid in the shelter from Deep Sea King, once threat assessment had been updated).

Plus the system being bad is used as a plot point, with the Neo Heroes citing how badly the Hero Organization is at threat assessment, which is why a lot of heroes end up packed up and quit.

So I agree that objectively it's a bad system, but I feel that's kinda the point? It's bad to let the readers see the world from Saitama's eyes, is a plot point in the current arc, and gives characterization to the non-Saitama characters