Hans-Georg Moeller gives an exegesis of Luhmann in Luhmann Explained. You will definitely find a critical examination there. Be forewarned though, Moeller is an orthodox Luhmannian so he does not diverge from Luhmann in any substantive way.
For what it's worth Law as a Social System uses the legal/illegal distinction. Religion as a Social System uses immanent/transcendent. Education uses pass/fail. And so on for Art, the Mass Media, Politics, the Economy...
The binary code (or opposition) can be most usefully thought as the distinction that grounds (my word) a functionally differentiated social system. I think it may not even have to be a strict binary distinction but mark end-points in a continuum which gives us (in the case of Law as a Social System, for instance) legal, quasi-legal, illegal, and their variants.