No, I don’t configure using nix, but partially because I don’t use home-manager. So I just write in the native config language.
The other reason is that I don’t believe configuring everything in nix is sustainable. You’re adding another layer that needs to be maintained by someone, and there isn’t much benefit. The native config is as much part of my configuration as a pure nix configuration would be.
And it doesn’t really matter whether it’s dedicated files or just inline in a nix module. I decide based on complexity. My neovim config is spread out over many files, but all other configs are inline.