There were 5 main issues on which Mgr Lefebvre challenged the proceedings of the Second Vatican Council: -
(a) The Vatican Agreement with the Soviet Union not to condemn Marxist Communism;
(b) Collegiality (which was being used by the Rhine Group & followers to attack the Papal Primacy), - in fact in many exchanges, Mgr Lefebvre & followers have championed the primacy of the Papacy against those who would downgrade it.
(c) Ecumenism: the final paragraph of the document does lay out very clearly the true Catholic doctrine, but many passages in the body of the text have been used to promote the idea that we now no longer strive to convert our non-Catholic bretheren. I think that the SSPX have been urging a more explicit statement of the Catholic position - and
(d) Religious Liberty (which was being promoted during the council in a form previously explicitly condemned). And also
(e) Mgr Lefebvre always maintained that the tension between ‘Doctrinal versus Pastoral’ led to endless confusion during and after the Council.
I do not think that the issue of the Liturgy was very high on the agenda at the time: the text of the Novus Ordo Missae, 1969, was very considerably different from that apparently being suggested in the appropriate Vatican II document. Note that, at first, Mgr Lefebvre was in favour of some modifications to the 1958 version of the Liturgy, and accepted the 1962 revision.
FAQ: Are the SSPX not schismatic?
Well, if the SSPX had proclaimed doctrines that were contrary to Catholic teaching, and after having been reprimanded, had 'pertinaceously' refused to recant, they would be guilty of heresy. If they had denied the primacy of Peter, either by word or by invading his jurisdiction (such as appointing a rival bishop to a diocese) they would be guilty of schism. What they actually did was to refuse certain commands, stating that it was out of obedience to the Faith. But this takes the discussion away from your main point, which was the objections of the SSPX to Vatican II.
numealinesimpetar