NixOS is a terrible solution rather than a reproducible one. Adding a layer of complexity to simple text files that can be easily edited and having thousands of links that are links to other links is just stupid. Filling up disk space like crazy and using more machine resources on trivial tasks is not an elegant solution. Editing configuration.nix or any other module, reading all the idiotic variable definitions that were invented to change a simple variable in a text file, is boring.
I don't know what the Nix designer (NixOS) had in mind when they thought that putting layer upon layer of complexity was a great solution. If something goes wrong in NixOS at the bottom layer, the nixos-rebuild command will produce weird errors. NixOS is an effort to make Linux complex and, in my opinion, useless. I could go on listing the shortcomings of NixOS, but I'll stop here...
I feel you (like many people) got burned by the steep learning curve. Empirically some pretty high powered companies use nix successfully. It's of course always difficult to know the counterfactual (would they have been fine with ubuntu) but the power to get SBOMs, patch a dependencies deep in the dependency stack, roll back entire server installs etc. really helps these people scale.
nixpkgs is also the largest and most up to date package set (followed by arch) so there's clearly something in the technology that allows a loosely organised group of people to scale to that level.
The problem is, once you have to wrap CUE, the loss of flexibility within a special-purpose language like CUE is enough for people to ask why not just bother writing the scripts in a general purpose language with better ecosystem support. And that's a hard sell in corporate environments, even ones that find benefit in type safe languages in general, because they can just pick a general purpose language with a static type checker.
I was initially most excited by cue but the novelty friction turned out to be too high. After presenting the various approaches the team agreed as well.
In the end we used jsonnet which turned out to be a safe choice. It's not been a source of bugs and no one complained so far that it's difficult to understand its behaviour.