Assuming you are careless enough to use only one mixer for a transaction, they form points of centralization. Assuming no mixer reputation, they allow a party

Assuming you are careless enough to use only one mixer for a transaction, they form points of centralization. Assuming no mixer reputation, they allow a party to steal your coins. Note that breaking a large transaction into small pieces allows you to determine a sort of short-term reputation while only risking one piece at a time. My standard first two steps used to be SharedCoin->BitcoinFog, for example.