Adjusting Left Sidebearing when adjusting metrics only works in multiples of 10 using CTRL + SHIFT + Left Arrow. It probably should work on a per unit basis using CTRL + LEFT arrow.

It does work for single units.

Sometimes there are some system wide shortcuts like for Spaces that interfere. You need to change the shortcut for that in the system preferences (or disable Spaces).

Cool, I look that up, thanks!