I think the backlash comes not from the addition of female characters in and of itself - even leads. SW had always been pertty heavily male.Seems, though, that with 8, they tried to take it from heavily male to balance in one shot. Short of Benicio, it seemed like EVERY new character was female. One of the first pilots you saw in the first fight - female, Billie Lourde getting prominence (though she was in TFA, barely), Maz (rfa), Phasma (rfa), Holdo, Rose, Rose's sister, there was another lady on the bridge as well, etc. Short of Benicio, i think the only other new male with a line was Justin Theroux, with one short line and a First Order officer or two. Follow all that up with the two top characters of the Resistance are Leia and Rey.
I don't find it problematic in general, but i can see where some people take issue.
So, now, your main characters of relevance for the heroes, who we're supposed to identify with are: Leia (who won't be in 9), Rey, Rose, Maz, Billie Lourde, Poe and Finn. So it can be taken as a bit of a tidal shift. Frankly, i didn't realize they went as far as they did until i wrote this out