Now I'm just a stupid ol' person who hasn't played dots for a week now, due to massive gaming on my end, and I certainly don't like facing Momiji too much(unless I'm Koakuma or Merlin or something), but I've never had too much of a problem with Aya or Youmu or Hatate. Youmu is entirely physical, picking up a Myon or something else high armor really invalidates her, and 3D escaping yourself(or her!) makes her cry. Aya and Hatate suffers from a similar problem, although as you guys said, mobility is high with these two which helps their game tremendously.
Instead of me rambling on, I'm just going to put it like this. The heroes that people favor right now are particularly weak to these three. There are a slew of characters that are also strong against these three, but nobody picks them because they'd rather also pick similar characters.