I think it depends on the overall skill level of the players and pitchers.
I know of one league on a nearby native american reservation that ran an "open" FP league, meaning open to any players regardless of gender. The pitching skill wasn't greeat, whether it was a male or female pitcher, and neither was the skill level of the position players. The league was mostly for recreational purposes and seemed to work.
There is also a city league not too far away for Men's FP that allows female players. One team had a femal that played second base. Again, the level of pitching in the league was not stellar and she held her own at bat. Another team had a female pitcher that was competitive, but was usually hit pretty hard by the male players.