em is relative to the font size of the nearest parent thus you can have situations where your font size of an element node is radically different due to the parent it is placed in. This can of course be powerful if used correctly, but it takes someone really good to use it correctly and perhaps it shouldn't be done in big teams because big teams imply some people not very good in some particular discipline.

on edit: adding this link https://j.eremy.net/confused-about-rem-and-em/