I think all bosses / champs should be immune to all CC in general. Works in other MMOs, don't see why it wouldn't work here.
Interrupting is kinda meh in GW2 since we don't have enemy cast bars or even proper wind-up animations, would take a lot of development to solve this.
The risen giants in Orr are quite well done I think; they are big enough so you can see their animations, and their attacks all have proper wind-ups that you can react to.