Pep and Xavi were products of respective teams so they knew exactly the pressure that comes with the job. They also were hired to play the "Barca way".
Zidane was an interim coach that exceeded all expectations.
United is much harder assignment simply for the pressure a manager has to face. Now if you were to say that Carrick should have a shot, I would agree, simply because he "grew up" at our club. He knows the pressure the managers face here. And unlike McKenna, I think he has the authority and respect of the dressing room