Cardinal Mueller has said publicly that the SSPX are “schismatic”, and he also made it clear that this was not an official determination and declaration of schism, but rather an assessment of their positions viz a viz the Church. And the Cardinal is not a “loose cannon on the deck”, firing off at will. He is not head of the dicastery because he is casual with statements.
And less anyone go off about the Cardinal being a liberal, he just took on, openly and explicitly, his current problems with the LCWR.
His comment: "as reported in the Corriere della Sera on December 22, 2013 : “The canonical excommunication of the bishops for their illegal ordinations was revoked, but a de facto sacramental excommunication remains for their schism; they put themselves out of communion with the Church. After that we are not closing the door and never will, but we are inviting them to be reconciled. But they too must change their attitude, accept the conditions of the Catholic Church, and the Supreme Pontiff as the definitive criterion for membership.”