This seems to be a false either-or because FB is probably not holding 100% reserves as actual paper cash in a giant Scrooge McDuck vault underneath their HQ. It's holding them at a bank, so that it can get the interest payments.
Thus, a user faces systemic banking system risks plus all firm/stablecoin-provider risks. That combined risk will almost certainly be strictly larger than the systemic banking system risk you'd face by just depositing funds in a bank account that you directly control.
Thus, a user faces systemic banking system risks plus all firm/stablecoin-provider risks. That combined risk will almost certainly be strictly larger than the systemic banking system risk you'd face by just depositing funds in a bank account that you directly control.