i have money in fidelity and vanguard because they're "too big to fail". there's a very small chance you'd lose a percent or two, but the fed absolutely would not let them fail completely due to the fact virtually every person has money in them directly or indirectly.
these money funds are paying about 1% versus 0% for treasury money funds (the safest around), so even if you lose 1% on a break-the-buck event, you basically broke even.