Hi! 
As far as Pinephone-style devices go, Mobile NixOS will at some point provide an appropriate user-friendly method to get a “foothold” on their devices. I would argue that while technologically they’re not really different from SBCs and other devices, they actually are in practical ways.
As far as other devices go, it depends. The general gist is “no”. But this is a good thing! Hear me out.
In planning for a better NixOS on ARM, I have stated explicit goal posts where we should strive to ship no device-specific images ever. Rather, we should strive towards standards-based boots, and work towards abstracting over the more subtle differences across common ARM platforms (mainly SBCs here).
There is one caveat though. This goal is more aimed towards generic interactive uses. For “embedded products” usages, like the PineCube, there is space for “highly integrated” product-like images. Though, as NixOS is a general purpose distribution, NixOS itself shouldn’t focus on such “product” images. People and group of people, in my opinion, should instead work towards producing and maintaining good turnkey configurations for these, in their own project-specific repos.
In other words: your question is far too encompassing for a single answer. The class of “product” really dictates the answer. 