Why is Silver Fang rated as S-class Rank 3 hero? It's clearly that there is bunch of heroes are more powerfull but for some reason ranked below. Zombieman, Flashy Flash, Child Emperor - all of the guys allready beaten monsters ranked above Demon level, while Bang along with his brother Bomb couldn't handle exausted Garou.

Is the ranking system not reliable source for heroes strength comparison or Bang just still didn't show all of his abilities?
As an addition to Pablo's answer:

The following page appears in Chapter 72 of the webcomic (text spoilers after):


   


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   Just before Bang basically one-shots multiple Dragon level threats in a row (he also takes out a whole bunch of Black Sperms in a flash), Bomb informs Genos that the reason Bang brought him (Bomb) along to deal with Garou was so that he (Bang) wouldn't accidentally kill Garou.  He was not there because Bang was not strong enough to deal with Garou, but because Bang was too strong and has problems not killing things when he's actually going all-out.