Even for things like trying out a new shell you can temporarily move the dotfiles somewhere and restore them back and it still takes less time than converting everything to Nix.
If you just want a throwaway VM, it's straightforward to create one through the UI cloud console. Whereas, terraform is nevertheless still a useful tool to use to manage VMs.
For stuff like installing development dependencies.. it's maybe not difficult to copy-and-paste instructions from a readme, but solutions like devcontainers or Nix's development shells can be useful even if costing more overhead.
My biggest complaint is what I mentioned above: it’s trying to be everything for package management, and adds a lot of complexity (and I disagree that it’s always necessary/inherent) compared to just installing a tool and sometimes upgrading it. That complexity often means I have to debug it rather than the tool that I want to - I might have to debug Nix instead of Node, which is not always straightforward. In my limited experience Nix got in my way more than I’d like, and in ways I didn’t expect or want to deal with, and until it’s as seamless as something like Homebrew or apt, it’ll be a hard sell.
Although you're right about nix's DX being quite rough, the problem isn't exactly that it "tries to be everything for package management".
Consider the assumption Nix wants to make about its packages: it should be possible to package software by putting it in some arbitrary directory (i.e. not just /usr/bin), where its dependencies are also put in (or symlinked to) some arbitrary location.
I think with well-written software, this should be a reasonable assumption, but you're going to run into friction with that. (Friction which will require you to have a good breadth/depth of understanding).
In my experience, a lot of the complexity when dealing with Nix is with the large and organic complexity of nixpkgs.
The "trying to be everything" is more incidental to the expressive package management. -- NixOS is 'just' an OS built upon the package manager; dev shells are 'just' shells which expose the build environment of a package; etc.
A simple UNIX script or PowerShell utility takes care of it.
None of the ones I have used during the last decades has ever grown to more than like 20 lines of code, minus comments.
I think the closest mainstream UX for "you can try out this program without having to install it" is running a Docker image.
:) I'd say Nix is second best at everything related to packages.
One thing I haven't tried yet is building a container from a flake, which would have obvious benefits for reproducibility. Still don't think it would help with service orchestration though.