This is not surprising given who he is and what his positions have been over his career. Most of the orthodox criticisms against cryptocurrency  I covered previously in my "This Time is Different: What Bitcoin Isn't" and "What Bitcoin Actually Is" series. But it's worth recounting how one could easily take many of these criticisms against Bitcoin, search and replace "bitcoin" or "cryptocurrency" for "US dollar" and come out with are more applicable criticism of the modern fiat money system.

"Cryptocurrencies, by contrast, have no backstop, no tether to reality. Their value depends entirely on self-fulfilling expectations — which means that total collapse is a real possibility. If speculators were to have a collective moment of doubt, suddenly fearing that Bitcoins were worthless, well, Bitcoins would become worthless."