Indeed there is no proof at all that circulating tokens are fully backed by USD and thus the fractional reserve banking theory Tether is accused of can't be debunked so far. The mere fact that they aren't able/willing to provide an official audit despite their promise to do so, makes it all the more suspicious. It has been the only functioning stablecoin for the longest time though and due to its liquidity quite useful and convenient on most exchanges. But because there is practically no way of cashing out USDT tokens at Tether for regular users, a real bank run seems quite impossible. The market price could however completely implode and we already have seen the first crack in its reputation with the recent Tether crash where it lost its dollar peg for quite some time and has a hard time to recover. I would only use USDT if I absolutely had to and for the shortest amount of time have funds in it. There are other stablecoins available now if you wanna store against USD value for a bit longer.