As far as why she ranks higher:
5* / Magic / 25 / D / 150 / Slow 5
26950 / 32100 / 41300 // 100350
5* / Magic / 20 / D / 150 / Slow 5
28900 / 20850 / 15650 // 65400
Mihiro costs slightly more to deploy but is otherwise similarly structured, and has a ridiculous advantage in total attack stats over Emma. Even discounting Sea targets, which are relatively uncommon outside of certain event bosses, the huge disparity in Air attack more than makes up for the difference on the Ground.
About the only times that Emma could be slightly preferable to Mihiro would be:
* if you need a Slow5 unit on a level that's strictly Ground targets
* in a Colosseum-type event in which you need those five extra deploy points for other units
...but even then Emma is a less than ideal choice, as pretty much every other Slow5 unit in the game has a stronger ground attack. Aleister, Stefan, Laura, Edgar and even Marguerite beat Emma with those narrow conditions.