Disingenuous interpretations of the evidence and stating things as facts that are very much still debated or denied by other experts.
A good example: Graeber posits that the European Enlightenment came directly from contact with Native American ideals rather than being a home grown movement. To support this, Graeber repeatedly references a book written by a French Army officer named Lahontan about his travels in North America in the late 1600s[0]. In this book Lahontan has dialogue with a fictional Native American named Adario that is more or less a disguised critique of European society. Adario bears similarities to Iroquois Chief Kondiaronk[1]. It's thought that Adario was a literary device for Lahontan's ideas but Graeber makes a very hand wavy argument that Adario was actually Kondiaronk and the dialogue was real. Graeber then uses this as his main piece of evidence to support his theory about the origins of the European Enlightenment.
I couldn't finish the book because I kept looking up the evidence Graeber was presenting and it usually ended up like the Kondiaronk situation.
Graeber is also very condescending when he's writing about ideas held by other anthropologists (like Jared Diamond), it was off-putting and came off as unprofessional for someone who was supposed to be a leading academic.
[0] https://en.wikipedia.org/wiki/New_Voyages_to_North_America
[1] https://en.wikipedia.org/wiki/Kondiaronk