Bitfinex and tether are likely doing a lot of illegal activity. No one knows how long it could take US to force a shut down, but in this event the value of tether would likely crash to almost zero.
I don’t think exchanges have added risk by using tether pairs because I doubt that they hold any tether themselves. Exchanges make money primarily by customers exchanging, NOT by the stability of the pairs they offer.
This is not financial advice but personally I sold everything after the subpoena news (luckily for a nice profit) and I think anyone with money still in the market after the subpoena news is at serious risk.