Most likely situation, if this were to happen: DOJ swoops in, seizes/restrains bank accounts, presses civil case to recover assets... then after several years, account holders get paid pennies on the dollar out of a remission fund. If you feel safe with fiat on the exchanges, don't. (Same goes for BTC, obviously)
BTC is a little safer since it only needs an internet connection to withdraw it, not a bank. It's definitely true you need to be wary though.