While it's impossible to say whether literally every single line of the show is untrue, there is zero support for any of his substantive claims among subject experts, and a vast array of evidence against them. You may be interested in this recent thread, with contributions from u/CommodoreCoCo and others.

The main problem I have with GH is that he doesn't really give any evidence for his claims, only a lot of "what ifs". He also doesn't seem to know of Occam's razor, the principle that with competing theories or explanations, the simpler one is to be preferred. For simple questions like "how did the people know how to build similar pyramids across the world?" his answer is always "it was taught by a more advanced civilization!", but this doesn't really answer the question, does it? It only raises more questions, like "why don't we have any evidence of this culture" or "how then did this more advance culture learn to build them?" Is simple answer is that pyramid is pretty much the easiest shape to stack rocks.

Also, he doesn't seem to understand how science works. He claims archeologists oppose his theory, because it's "an attack against the current paradigm, and archeologists are reluctant to change the paradigm", but that's simply untrue. The paradigm changes constantly every time new evidence is discovered.

Now, I'm not a historian by education, merely a hobbyist, but I am a physicist, and we have our fair share of similar people to GH. I can't speak for archeology, but physics is a field that's very open for new ground-breaking discoveries. Still, there are plenty of people with their own theories claiming "mainstream scientists" oppose their theory because it's "too dangerous for the current paradigm". It's always simply because they just don't have the evidence. With archeology, I'm sure Hancock's claims would be accepted, if he would simply show the evidence.