Regarding licenses, most (all?) packages in the official nixpkgs repo have license metadata, and you can set your configuration to forbid/allow proprietary packages (the "allowUnfree" option), or use a whitelist (e.g. forbid everything except Flash).
Guix is a GNU project, and doesn't allow proprietary packages in their collection at all.
Guix is a GNU project, and doesn't allow proprietary packages in their collection at all.