I have noticed an issue on my 2009 iMac where the screen brightness controls no-longer work. The screen is stuck in a permanent 'middle' brightness regardless of the settings.
If I boot into 10.11.6, the brightness controls work as expected, but as soon as 10.12.5 starts, the screen dims or brightens to a fixed 'middle'.
I've had 10.12 installed on this for a very long time now and this used to work until, I think, 10.12.4.
Apologies if I have missed this discussed, but any help would be appreciated