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.
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.