I believe authorities can still freeze any exchange wallet if illegal activity is reported regardless on the pertext of investigation. Exchange can dispute or deny the request to freeze any wallet but generally they do not as they do not want to complicate their lives.
It's not the authorities you have to worry about, it's the people issuing the stablecoins. Most of them will go hysterically overboard to look legit in the eyes of the law. And it's not just law too, Gemini have refused redemptions to 'protect our position on Coinmarketcap' so your money is worth less to them than having a nice large number on a crappy website.
