I'm guessing one of the major reasons more packages are "broken" for macos is there is a CI system running for linux/nixos (travis) and not one for macos. So macos breakages can slip in easily. If a passionate mac advocate wanted to take on the responsibility of setting up & running a CI machine and hooking it up to the github PRs, I can only imagine it being a good thing...
the major reason for poor package support is low number of osx users (hydra - nix build farm - builds binaries also for osx).
At the moment that's broken (the 10.9 SDK is installed in 10.11, then links against some 10.11 libraries, which is blocking mplayer). I don't have the knowledge to fix it.
homebrew and fink have had the same problem (deciding when to link against system libraries and when to try to rebuild) -- hopefully nix will get over this hurdle.