Foreword: In this question, I ask NOT about default risk.
Abbreviate 'money market mutual funds' to MMMF. MMMF are a type of mutual fund, which is always riskier than cash, per this MoneySense article dated Dec 4 2012, with which I agree:
[...] Since mutual fund prospectuses clearly state that the value of the units of mutual funds will fluctuate, no mutual fund company is under any obligation to make good your loss should the money market fund’s unit value slip below what we’ve come to think of as the norm. [...]
I am perturbed instead by the quotes below which imply that MMMF are as safe as and equate to cash, and so which conflict with the foregoing.
Source: Investing For Canadians For Dummies, 3 Ed (2009) by Tony Martin, Eric Tyson
[p 16:] Cash equivalents are any investments that you can quickly convert to cash without cost to you.
Money market mutual funds are another type of cash equivalent. Investors, both large and small, invest hundreds of billions of dollars in money market mutual funds because the best money market funds produce higher yields than bank savings accounts. [...]
[p 39:] Another good choice is to keep your liquid savings in a money market mutual fund. These are the safest types of mutual funds around; for all intents and purposes, they equal a bank savings account’s safety. [...]