Although it isnt shown in the show at all or at least there is only small hints of it, i think Mihi's biggest problem is she overworks herself to a fault to her health and i worried about her throughout the show. She beats herself up so much whether she does good or not it seems. I dont know where Mako gets her strength but even though she stresses so much about her performance she is able to not let it affect her health too much it seems. I could be wrong but thats my view on the differences between them two, but we all know they're both aces.