Users should be concerned about their fiat deposits on Binance.US because their TOS states that it is 100% held in Silvergate and Signature Bank.

The TOS states: All U.S. dollar deposits associated with your BAM Fiat Wallet are owned by you and held in an omnibus account for the sole benefit of customers at either (i) Silvergate Bank, a California state-chartered bank that is a member of the Federal Deposit Insurance Corporation (“FDIC”) or (ii) Signature Bank, a New York state-chartered bank that is a member of the FDIC (collectively, the “Sponsor Banks”)....

TOS here: https://www.binance.us/terms-of-use

They have not made any statements on this apart from downplaying the issues of ACH withdrawals that users are experiencing. According to them only "a small subset" of users are affected but this sounds pretty suspicious considering how they have pretty much lost all banking access in the US.