See my edit. What protects you from the fund being mismanaged?
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.