While Judit Polgar dropped out of women's chess before she was even a teenager (excepting one final women's olympiad at the age of 14), Hou Yifan has played in both women's and open events in her career. She has won four women's world championship events and has utterly dominated women's chess in the wake of Polgar's retirement from the game in 2014.
While there are other strong women who can compete with her, albeit as heavy underdogs in a longer match, Hou's toughest opponent seems to be FIDE. While the open world championship eliminated the knockout system after Tripoli in 2004, returning to the older approach where the champion plays a match against the winner of a challengers' cycle, the women's world championship does not work in that way. Considering that Hou Yifan has been dominant and generally outrates her closest competitors by around 100 points, this is rather odd.
Hou went along with this for a few years, going through all the machinations to keep qualifying for the next world championship match, but she has decided not to do so any longer, at least not until FIDE changes their procedures. On the bright side - for her, anyway - it means she can focus her energies on "men's" events, where she is the one trying to break through to the next level rather than trying to beat back the hungry hordes.