The goal is to get pleasing face reconstruction, everything follows from that. There's no objectively correct loss function, it's something that is selected because it has a good effect.

A simple example could be sensitivity to color differences: with lighter skin tones there is a fairly large difference between eyebrows and eye features and the person's forehead or cheek. Someone with a pretty dark complexion might have features that are distinguished in a very compressed set of colors. Depending on the loss function a dark complexion face could be essentially flat and featureless.

This could be fixed of course... but it requires changing the loss function.