Well the main one is that he posits there was a broad-reaching civilization on Earth that was effectively wiped out at the start of the last ice age, about 12,000 years ago.
Typically the Mesopotamians, around 6,000 years ago are considered the first civilization. So it’s not likely to get any support for the idea of “civilization” before that. Most of what a civilization had is usually wiped out after a millennia or three. (Or deliberately destroyed, thanks The Pope.) Connecting them is difficult, but he finds interesting parallels, examples, possible ways it could have happened, etc.
One of those ways is through paleoastronomy, which upsets basically everyone and their dog, but it says essentially earthworks created to point to or mirror astronomical features can tell us things like when they were built. Or, with enough like-examples suggest a common theme or purpose.
He’s engaging and intelligent, and pretty laid back about the wild theories which makes some people really dislike him. Here’s a set of specials he did in the 90’s? I think? Which are pretty thought provoking if you’re into that kind of thing.
What I don’t buy is that all of history has been discovered, correctly interpreted, and sewn up with a bow on top. But I understand he’s particularly annoying to academics because he wants to be an explorer, an educator, a theorist, an archaeologist, an anthropologist, an astronomer; and yet he hasn’t completed coursework or any other feats of strength to prove he knows anything about those things. Skeptics just hate him immediately on the face of everything. Which, yeah. That also makes sense.