Any downside to "tax-exempt" money market funds like MCSXX or FFTXX?

I'm wondering just generally if they come with more risk than the treasury or repo-backed ones. Does the NAV stay reliably pegged at $1? Do they have more uneven returns because they are muni-backed? But if you want the context, you can keep reading: I currently use TTTXX for my money market fund, because it is state-tax exempt. However, I'm in the 32% marginal federal bracket, 9.3% in my state, and my gains are subject to NIIT as well of 3.8%. The tax-exempt funds MCSXX and FFTXX both yield ~2.