We have public financials as of earlier this year as a private company, so you can review our liabilities. https://river.com/financials
We are not downplaying custodian risk, we are encouraging awareness of custodian risk by telling Bitcoin users to demand Proof of Reserves from their exchanges.
Even if you withdraw to self-custody, you still don't want to take the risk of an exchange not being fully backed before you get the funds out, and you don't want to be recommending an exchange that may be selling paper bitcoin to friends and family who may take a longer time before they get to self-custody.