Well, I actually enjoyed contributing to NixOS, but running it on a router is certainly not a "it works out-of-the-box" experience. Even if you are building a simple SOHO router you need to know a lot more about the implementation details (which daemon does what, how to write firewall rules, how to setup the network interfaces, etc.). This is because NixOS is general-purpose OS, unlike VyOS or openwrt.