I linked Wikipedia, but to summarize some of his questionable claims:
* There were plans under consideration in 2006 for a nuclear first strike on Iran.
* Osama bin Laden wasn't killed by US Forces in a raid in Pakistan
* The Syrian gas attack wasn't done by Syrian government forces
* "I don't necessarily buy the story that Bin Laden was responsible for 9/11"
* That the Skripal poisoning wasn't done by Russian forces.
Lots of these views have significant backers amongst conspiracy theorists. But unlike some of his other stories (say in the same time frame) there has been very little to no evidence showing he is anything like correct.
Basically, post 2000 the only one of his stories which has turned out to be well supported was Abu Ghraib.
Classified info takes time to be released, so I wonder if the fact that his recent reporting hasn't been confirmed is more a function of freedom of information / declassification timelines than proof that he has dropped the ball.
It's interesting you question his record, I thought Hersh's record was excellent; which major stories are you referring to? With the kind of reporting he does, I would have expected many more major mistakes to be made but he seems to make them very rarely.