There's a section of the FAQ dedicated to Dan Carlin and his Hardcore History.

I think the threads in the FAQ by /u/Quitchy do a pretty good job describing it. I particularly like the way /u/Georgy_K_Zhukov put it in one of his comments, Carlin “is an entertainer doing history, not a historian doing entertainment.” It’s quality, well sourced pop history. But it’s definitely pop history. I’m no expert on any of the subjects he’s covered but I’ve caught errors before; for example Gavrilo Princip wasn’t going to lunch and having a “consolation sandwich” when he shot Franz Ferdinand. And Carlin loves to “compare and contrast” in ways that clearly become ahistorical. He also loves his metaphors. I’m not sure his Planet of the Apes references are as good as he thinks they are.

Like I said though it’s usually well sourced pop history. He always provides a bibliography. There’s 33 cited works in the bibliography for his episode Celtic Holocaust. If you read all of those you’d be fairly well versed on the subject. If you could remember it all and understand, it’d certainly qualify you to comment on threads on that subject here in /r/AskHistorians. When Carlin is sticking close to his sources and citing them word for word, he very good. When he’s mixing in personal anecdotes and metaphors, he’s less good. But as historical podcasts go, it’s usually fairly accurate, and very well produced. And Carlin definitely has a great radio/podcast voice.