His reporting on My Lai was based on internal government documents. Abu Ghraib was already being reported by other sources like Amnesty International, so he was backing up by other reporting there.

Thing is, I'm not asking you to trust me. Not one bit. I'm asking you to apply an appropriate level of skepticism. Common practice for an accusation this serious is to get more people talking. But here, the whole accusation rests on one source. Why should we trust this source? Because Seymour Hersh said they know stuff? And since then, nothing. Now maybe Hersh is still digging and will publish something in the future. If so, I'm all ears. Until, I stay skeptical.

And it's not just me pointing to how Hersh uses anonymous sources as being problematic. His Wikipedia article has a lengthy section covering both criticism and defense.