Makes me think...
I believe everyone in the world has equal power, but that power is divvied up in specific ways - most of the worlds population are immune to ghosts/yokai whatever, but that means we can't interact with or see them in any form.
Then for the people that can, depending how much they specialize, other things get downgraded and Miko is just unlucky.
Character examples -
Romm is fairly powerful against them - but for being that good, means he can't see them very well (he says its like looking at glass window and seeing a faint reflect) but is completely susceptible to what they can do to him.
Mitsue feels more like an all rounder - she is decently strong, can see them better than most, and doesn't get affected by them on the level of Romm or Miko.
Seto - is extremely powerful vs them and doesn't seem to be able to be affected by them. He can't see them but can hear them.
Miko - Can see them extremely clearly, can hear them very well (although she doesn't speak the language, but that may be a skill issue) and they can interact with her as if she exists on their plane too, luckily they have to know she is for them to do anything to her. Problem is, all her "stat points" went into that, so she is utterly useless fighting them.
Yuria - lesser version of Miko, she can see them sort of and hear them, as well as see Auras, but because its so much less than Mikos, she is practically immune to the interactions since she is like.. barely into the esper territory
Of course outliers I really cant explain, Michiru and being possessed, Hana's spirit being so strong that it can replenish life force or being used as a kind of food.