Well, they are not 100% in UST, and I don't know I would call even their UST reserves "backing", as those UST could be seized with one signature from a judge.

I personally trust holding Tether for the duration of time that it takes me to exchange my BTC for USDt on one exchange, transferring that USDt to another exchange, and using it to buy back BTC on the second exchange (at a cheaper price, ... i.e., performing arbitrage). And not one second longer.