Money market funds and short term t-bills are basically always liquid at face value, unlike longer term bonds. They fall below investment value maybe once per 50 years and that usually lasts a couple days.
https://www.sipc.org/for-investors/what-sipc-protects
https://investor.vanguard.com/investor-resources-education/m...
And frankly if that's the case I wouldn't be betting on FDIC or equivalent insurance actually working anyways.
But even "less" secure ones are heavily regulated to be kept at 1$ of NAV and SPIC backed.
Convenience is easily worth it.