Small nitpick, they aren't symlinked to their original position, but instead to a "profile" (global for instance is in /run/current-system/sw, your local profile is $HOME/.nix-profile).

This has several consequences. It means binaries distributed with the expectation of libraries being in their original position (e.g. /usr/lib/libc.so) don't work as is. It also means scripts depending on the presence of /usr/bin/bash are broken (which happen far too often).

I love NixOS, but that is one of the biggest annoyance I have with the system.