Oof, I was just talking about making things declarative there. If you want to configure it the old-fashioned way, like you would on other distros, then those difficulties don't apply.

In more general terms, though, it's a bit of a double-edged sword. The Nix package repository has more packages than other package managers: https://repology.org/repositories/graphs

So, the chance of finding an obscure software, that's already packaged, is rather high.
Here's the online package search, if you want to check the availability of some of the obscure software you use: https://search.nixos.org/packages

But then, yeah, the flipside is that, from what I understand, you can't just download a random executable off of the internet and run it, because of the Filesystem Hierarchy Standard not being adhered to, as the post also mentions.
You can set up Flatpaks, and I believe AppImages would work, because those also live in their own FUSE filesystem. Well, and there is ways to emulate the FHS layout to get normal applications to run, too.

But yeah, way out of my field of expertise there. I have only one software installed which isn't packaged for Nix, which is a program I wrote myself.
And to get sufficient FHS emulation for that, I just needed this line in my config:

programs.nix-ld.enable = true;

More complex programs will need a bit of extra configuration: https://wiki.nixos.org/wiki/Nix-ld

(I could also add a flake.nix file into my software's repository, though, which would make it so it could be installed straight from my repo, as if it was packaged.)