That's definitely true, but maybe I've just been lucky, pretty much every proprietary program I've wanted to install in NixOS has been in Nixpkgs.
Skype, Steam, and Lightworks are all directly available in the repos and seem to work fine as far as I can tell. I'm sure there are proprietary packages that don't work or aren't in the repo, but I haven't really encountered them.
I've unfortunately encountered a few. TotalPhase's Data Center software for their USB protocol analyzers is my current annoyance, someday I'll figure out how to get it to work but thus far it's been easier to just dedicate a second laptop to it.
I am hoping that Flakes will work to fix this problem somewhat; at least in theory it can be a situation of "get it to work once then it'll work forever", and then trivially distributed later (even without a blessing from Nixpkgs).
I'm honestly curious what packages you have a problem with