A current concern is that falling interest rates will cause more homeowners to pay off their loans early by refinancing. That could crimp funds that own Ginnie Maes, Fannie Maes and other mortgage-backed securities, because it implies shareholders will get a premature return of principal and thereby face a reinvestment dilemma. The point is that bond funds can be confusing. They aren't sold with sales charges attached, but many bond portfolios are. Money funds are much more homogeneous than bond funds, which means you can spend your research time elsewhere.
Money funds invest in short-term IOUs issued by corporations, banks or government agencies.
Like certain bond funds, you can find money markets that pay tax-free interest. As noted, bond funds tend to beat money markets over time because the investments they hold pay higher interest. But if it's pure safety you seek, then money funds are the better bet.