This recent thread is an excellent start - as is this other thread and also this excellent thread

 

In short,

• He doesn't really give any evidence for his claims, only a lot of "what ifs"
• 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.
• His formula is unscientific;
• Hancock describes something cool in vague, romanticized terms. This is often done in the first person in a journalistic style to provide an air of legitimacy without needing to be thorough
• Hancock asserts the thing's mysterious nature. He does this actively by showing how things archaeologists said 100 years ago (or never said at all!) fail to explain the thing, or passively by ignoring decades of research, positioning himself as the first person to ask these questions.
• Hancock offers an additional, enticing observation that, having had all other context stripped away, functions as the single knowable fact
• Hancock suggests his kooky hyper-diffusionist explanation for that observation that only makes sense if the handful of observations he's provided are the only ones you know

Because Hancock has stripped away all context for his observations, he can make whatever claims he wants. And because most readers have no familiarity with archaeological literature outside their high school history books, they don't know how much information Hancock is not telling them.

 

More recently, Hancock has shifted to theories that violate that first scientific fundamental. His book America Before is the culmination of his obsession with the Younger Dryas Impact Theory.

 

Sensationalized unproven claims while dismissing science is absolutely bad and should not be encouraged.

 

^(Note: 95% of this comment isn't my own stuff, but copy-pasted from the linked threads)