I think it's important to be clear that the Federation isn't custodial because it's not letting you deposit as much as it's selling you an ecash token. That token has no ties back to the original UTXO you paid. The federation also doesn't custody the ecash notes it creates, each individual owner custodes their own ecash.

You can go back to the federation to trade your ecash for sats but there's nothing there to force them to redeem.

I think people selling Fedimints as depository products are doing everyone a disservice to what's really happening under the hood. I also think we'll see much more transparent mints with public guardians as time goes on that should help to alleviate some of the concern but we still need to be more clear what these products are.