A key thing they mention is they are not a bank. Your funds kept are FDIC insured but they're misuse of the funds are not. River has a good reputation, but anything can come in this space. Not your keys, not your coins.
Sure but you need to have them anyways in a bank. Bank A or Bank B, they're all at risk in that sense. This doesn't change that equation, I might as well have my TBill yields be converted automatically into ₿.