Linux

Well I guess it depends how deep you're in the rabbit hole already, I think it's relatively easy for me at this point to create a new package (I'm maintainer already for quite a few). But yeah ... steep learning curve ... Less so with Nix itself, though non-the-less, it's a simple functional programming language with a new paradigm (derivations). But rather NixOS/nixpkgs Nix magic. For example there's a dynamic dependently typed type-system built on top of untyped Nix in the NixOS module system that is spin up on evaluation time.

But I understand your point, at the beginning of my NixOS journey I have also rather created a "good enough" Dockerfile. Depending on the exact context I still do this nowadays (often because there's an official well maintained docker image in comparison to a not so well maintained Nix one, and the context is too complex to maintain/develop/extend it myself). But if there's a good solution in Nix I rather use that, and that is often less headache than setting up a service with e.g. docker-compose. I also use flakes mostly for a dev environment, if you're a little bit deeper in it, you can spin up a relatively clean dev env in short time (I'm often copy pasting the ones I have written from different projects, and change the packages/dependencies).

As I said it has a steep learning curve and documentation is pretty much the nixpkgs repo itself (well after understanding the basics of Nix and NixOS at least, with the combination of the https://nixos.wiki mostly IMO). It also takes some time to get used to the quirks of NixOS (and understanding the necessary practical design decisions of these quirks).

But I have nowadays seldom trouble with switching the generations (i.e. nixos-rebuild switch), unless you're updating flake inputs or (legacy) channels (where e.g. a new kernel might be used). In that case it makes sense to reboot into the new configuration. Also, obviously that can lead to short down-times (including just restarting a systemd service, if a service has changed in between the generations), if that is unacceptable, there obviously needs to be a more sophisticated solution, like kubernetes via e.g. kubnix. I'm not sure how much of that can be achieved with Ansible, as I haven't used it that much because I disliked the "programming" capabilities of the Ansible yaml syntax (which feels kinda hacky IMHO).

But apart from NixOS, one can also just use Nix on a different system to e.g. deploy or create docker images (which can be really compact, as only the necessary dependencies for a package is packaged) that in turn could e.g. be managed with Ansible or something...