Carlin spends a lot of time telling you where he gets information, who he's quoting from, how reliable they are, what the historical consensus is, etc. In a word, historiography.

That's not to say he never gets anything wrong, he and his researchers aren't steeped in the subject for their whole careers so they'll miss things. He is biased towards the exciting stories and sources, but usually qualifies them with "I'll tell you this fun story but historians disagree about X Y Z because of A".

I'll have to look into the criticisms but my impression just from listening is far better than any other pop history.