They aren’t insured: Money market funds are not eligible for CDIC coverage, so the money you’ve invested in a money market fund will not be protected in the rare event of a bank failure.
There’s no capital appreciation: Money market funds are considered effective at preserving capital, not growing it.
Returns are not guaranteed: Although it’s rare, money market funds can still lose money.
You may have to pay fees: Some money markets funds are professionally managed and carry fees that can lower your returns. Be sure to research the specifics of any fund of interest before investing.