Distro agnostic packages like flatpaks and appimages have become extremely popular over the past few years, yet they seem to get a lot of dirt thrown on them because they are super bloated (since they bring all their dependencies with them).
NixPkgs are also distro agnostic, but they are about as light as regular system packages (.deb/.rpm/.PKG) all the while having an impressive 80 000 packages in their repos.
I don't get why more people aren't using them, sure they do need some tweaking but so do flatpaks, my main theory is that there are no graphical installer for them and the CLI installer is lacking (no progress bar, no ETA, strange syntax) I'm also scared that there is a downside to them I dont know about.
Part of the reason is that people are still finding out about it, Project has no marketing so it grows organically, in the last year the number of contributors grew by 25 percent.
Another problem is that it still needs polish in term of ease of use, for example it takes me forever to search for packages using the nix-env command but using the website it takes less then a second, That's a basic feature that still does not work correct, Plus their documentation is still not great in my opinion, I actually helped improved it and the improvement they made is still not really good IMO.
You're actually supposed to be using nix search nixpkgs#packagename to search and nix profile install nixpkgs#packagename to install.
However, to use both of those, you need to have the "experimental" (not really though, most of the community uses them) features of nix-command and nix flakes enabled, which they aren't by default.
And of course, nowhere on the main documentation did I find any if that, I only found it via the pain of using it wrong, and forum posts.
Nix's documentation is horrific. I've had situations where I only got help via discord...