There's at least one customer out there who has an RPi with NixOS on it from me. Probably still works to this day. It was the best use of NixOS I think I've ever found.
I've switched up to NUC-ish hardware which WAS cheap until recently. All the components just a bit higher quality than the lowest-bidder you get on the Pi. Run nixos on that and you've got a really solid platform.
More probably worst use of NixOS. In embedded we care for every single kb, and don't have tolerance for this kind of space waste management system
Embedded means different things. Personally I have a hard time calling anything that runs Linux as embedded, having cut my teeth on microcontrollers.
If it's a custom Yocto thing or it involved buildroot at any state of provisioning it gets the moniker "quasi-embedded" to me. Doubly so if it never actually connects to the internet, unlike these water controllers, apparently.