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.
Point is that your BTC on exchanges are not real BTC. You cannot withdraw them unless they send them to you. If we're talking about an exchange "going rogue", they aren't going to fulfill withdrawal requests.
In fact, at least with fiat, you have a shot at getting something back via three-letter agencies. BTC, not so much.