They have yet to release an actual legal audit, by an official auditing company.
They may be fraudulent; they may be not. Regardless, they have a lot to lose by revealing their banking partners to the world. The United States probably used the Bank Secrecy Act to force large American banks to not send money to accounts associated with Bitfinex. If Bitfinex reveals its banking partners for Tether, then they risk getting cut off from those partners or those partners getting cut off from western banking.
Seems like no amount of verification will ever be enough for a good chunk of the crypto community. The scars of Mt Gox and BFX socialized loss are too deep.
The market seems to think otherwise, you can unload your tether for $1.02 most days if you want.
And I believe that if you want to prove to the world that you have a bunch of money then you should do a full, legal, audit.
That's the bar. If they have all the money, then a legal audit will prove them right.