This has always been a worry in the traditional stock market. With a stop loss order you're basically creating a "secret" order to sell at below market value. Anyone who knows about it has a direct incentive to profit from it.

Stop-losses are very important in investing but I'd recommend always executing them manually at market rate. Set a price alert to notify you. Of course if the exchanges are down you're sol, but if BTC is on a bullet train to $10 your exchange stop-loss probably won't fill at your price anyway.