One worry is the lack of an external anchor of value. Cryptocurrencies are no different from the dollar, in that they rely on people having a shared expectation of their utility. However, conventional money is also backed by states with a monopoly on force and central banks that are lenders of last resort. Without these, DeFi will be vulnerable to panics. Contract enforcement outside the virtual world is also a concern. A blockchain contract may say you own a house but only the police can enforce an eviction.