During a massive crash I have seen coins like USDT trade for 0.90 dollars, so they can be affected if there is an important crash in the price of bitcoin, however after the crash is over stable coins go back to their original value, so in a  way they can be a source of unexpected gains despite the crash as you can easily earn some money during those difficult times, however make sure the stable coin that you are buying is a reputable one as you do not want the same that happened to those which invested in UST to happen to you.