When you think about bringing it to production (eg getting dev teams to migrate to it), Nix goes from a genuinely interesting idea to an "oh, that's cute" experimental toy because no one is going to spend hours learning Nix's weird DSL. It's simply not approachable in its base form.
I spent hours converting my devboxes to NixOS and managing my dev environments with home-manager and I still don't have a clue how any of it works. Errors are opaque and annoying to debug. Dev environments constantly break and change in ways that belay the "reproducible" nature of Nix. If someone actively interested in Nix can't easily grasp it, how does anyone expect it to catch on in the real world?