One-Punch Man - How tough is Boros?

Oh, absolutely.

If Saitama went serious from the start, Boros wouldn't survived even one punch. But Saitama is never serious from the start and his weak punch still killed everything before and after Boros with just that weak punch.

My point is that while yes Boros has no change against Saitama's true power, he still survived a hit that killed everything else. Surviving a weak punch from Saitama is a achievement that so far only Boros was able to do.