Basically the worst thing that can happen to you in this life is: You put $10,000 into a money market fund, but then later find out that you only have $9,999. I mean, that's not really the worst thing. Really you just lost a dollar. It's not that big a deal! You'll get through this!
But money market funds are weirdly emotional territory. There are things that are "investments," some of which are riskier than others but all of which can go up or down in value. Then there are other things called "cash," which are just cash: They can never go up or down in value, though they might pay interest. Shares in money market funds are supposed to be cash, but are actually investments, and the category confusion leads to lots of really intense reactions. This summer the Securities and Exchange Commission announced some mild new "floating net asset value" rules, requiring some money market funds to tell investors when their shares lose a little bit of value. In response, the money market industry build actual barricades in the streets and stormed the SEC's offices with pitchforks and torches, it was pretty weird.