Well, Christ Himself said, “By their fruits you will know them.” The fruits of Vatican II have been disastrous, so I’m rather inclined to believe the SSPX’s rationale. Archbishop Lefebre was not the only one to say the Novus Ordo was theologically flawed. Cardinal Ottaviani and others wrote a majorly critical paper predicting the disasters that happened. The SSPX is not wrong. The Modernists just don’t like them pointing out the obvious, namely, that Modernism is heresy. Pope St. Pius X declared it to be so in 1907 and the SSPX is just abiding by the teachings of that Pope. Something is wrong when an encyclical by one Pope is totally disregarded by a subsequent Pope.
The problem is that most post Vatican II Catholics still think it's because Vatican II was just "ambiguous" and "unclear" and subsequently misinterpreted. Sound familiar?