Bishop Sanborn wrote an article many years ago condeming what he called "Opinionism".
He based his certainty of faith on the fact that the proposition ... "The Church cannot teach error" ... is known with the certainty of faith.
But his mistake was in not recognizing that when you break the entire thing down into its premises, some of these premises are arrived at through private judgment.
Major: Legitimate hierarchy cannot teach substantial error to the Universal Church.
Minor: Putative hierarchy taught substantial error to the Universal Church.
Conclusion: Hierarchy was not legitimate.
Yes, the major has certainty of faith (which many R&R deny). But the Minor does not have certainty of faith. Consequently, according to the logical "weakest link" principle (peiorem partem sequitur conclusio) the conclusion cannot have greater certainty than ANY of its premises. There are certain propositions in Vatican II which certainly appear to contradict previous Magisterium. But what if there's some distinction I'm missing? There are many in the Novus Ordo who have done various theological gymnastics to reconcile V2 with Tradition. Just look here on CI how "certain" some people are that their theological opponents are promoting heresy (e.g. my accusation that Stubborn is a heretic). [Ironically, the closest thing to heresy in V2 is their ecclesiology, and +Sanborn ultimately holds the same ecclesiology -- but I digress.]