Their website says: "USDC is fully backed by cash and short-dated U.S. government obligations". I'm sure they'd absolutely love an official audit of that, but given the murky regulatory environment for crypto, I'm also sure that Grant Thornton is refusing to do anything more than it already is.
I admit I'm biased, I've used USDC personally & professionally, and I know people involved with running USDC. They're not evil geniuses. And even if they were, you could make so, so much money by breaking it ala Soros & the Bank of England, that I think it would've happened by now.
The market cap of USDC is something like $52 billion. No bank or banks would allow Circle to hold all that in a commercial bank account, so they diversified into holding treasuries. I don't really see the issue with that.
I admit I'm biased, I've used USDC personally & professionally, and I know people involved with running USDC. They're not evil geniuses. And even if they were, you could make so, so much money by breaking it ala Soros & the Bank of England, that I think it would've happened by now.