Uh that's sad! There is actually an issue opened about NixOS; but, since I am not using it, it's pretty hard for me to debug.
If you want some help you can comment on that issue though!
There is also Clight (https://github.com/FedeDP/Clight) if you want automatic backlight adjusting given ambient brightness. It supports external monitors through ddcutil C library and has many more features to offer.
Disclaimer: i am its creator ;)
It actually has lots of improvements in the 6.0.0 pull request that i am delaying because i am too lazy (and haven't got much time) to fully rewrite doc.