It seems brightness does not update the brightness setting in the system. This can be observed from the display settings (System Preferences > Displays), the slider does not change even after quitting / reopening preferences.

This can also be observed on a Touch Bar, the position of the brightness slider does not update after running brightness.

Steps to reproduce:

1. Set brightness to max (in System Preferences)
2. Quit System Preferences
3. Run brightness 0.5
4. Check brightness in System Preferences, still at max

I think this is related to #20.