Well there's a complete lack of evidence for his claims, the fact he thinks it's Atlantis or an Atlantis analogue, which we know to be a fictional creation, the fact he's never done any actual archaeology, and the fact that we have lots of evidence of civilisations around the same time as his supposed ancient advanced civilisation, but they just lived side by side, and one left loads of evidence, and the one he supposes existed didn't leave any evidence at all.

Archeologists are constantly digging, making new finds, and publishing their results. You can find them on the internet. They're not hiding anything because their jobs count on them finding and publishing. No archeologist is hiding evidence of ancient advanced civilisations because they don't 'believe' in it, whereas Hancock believes it must be true, so they must be hiding evidence and of course the only reason he is criticised is because he's right and they don't want to be proved wrong.

Zero understanding of science and history.

Graham has been a laughing stock for decades now, not a single academic considers him to be even noteworthy. He had an audience of conspiracy theorists, and now, unfortunately, conspiracy theorists are more organized and you can make more money off of them so he's having a bit of a career resurgence. It also helps that his son runs a department at Netflix, which also explains why he's been giving a show despite possessing nothing that qualifies him to be an expert on any such topic that he's presenting. I wouldn't even trust Hancock if he told you were the nearest petrol station was. The man has nothing of value to add to academia.