Downsides of mixtes were how I finally convinced my wife to go to a standard frame for her latest bike:

1. On hanging type automobile bike racks, often annoying and/or harder to hang with other bikes
2. Harder or impossible to have two water bottle cages on top of down tube.
3. Harder or impossible to use frame bag.
4. You never wear a skirt riding, anyway.
5. Men's sensitive parts are more a risk from a closer to horizonal part bar.
6. Many if not most modern frames have "sloped-enough" top bars anyway.

Later on I upgraded to a platform-style rack and my wife said "Hey, wait a minute..." but by then she realized she liked the new bike better than the old "girl's bike" anyway.