That's a bit of a slippery standard, and potentially a double-edged sword.  In fact, the neo-SSPX point out the debacles with Fr. Pfeiffer, Pfeifferville, and Fr. Hewko as examples of the bad fruits of the Resistance.  SVs' good fruits are the same as that of most Trad clergy ... providing the Traditional Catholic Sacraments and exposure to the Catholic faith to the laity.  There are good SV priests, and some not-so-good ones.  There are some good R&R priests, and some not so good (CM exposed quite a few of the bad ones, and then there are Modernists like Fr. Paul Robinson floating around).  Nobody is perfect or pure here.

Ridiculous smear job and projection.  Some of us objected to how The Nine conducted themselves after their split, but there are those in the R&R camp who think that +Lefebvre is impeccable and infallible, infallible both when he says A and when he later says NOT A.  So both A and NOT A are true for the +Lefebvre cultists ... and thus the battle between some of them about which group is TRULY most loyal to +Lefebvre, because you can find quotes that The Resistance like and you can find quotes that the neo-SSPX like.  What puts it all into context is to understand the chronology, where in the early 1980s, +Lefebvre was optimistic and very amenable to a practical agreement with Rome.  At that time, The Nine split off for many of the same reasons that The Resistance object to the neo-SSPX orientation.  Only difference is that it was a temporary shift in +Lefebvre's thinking, and by 1985, he was solidly against any such cooperation with Rome ... whereas the neo-SSPX direction seems pretty set and not liable to change course, not to mention that the neo-SSPX have had the benefit of many extra decades of hindsight.  When Wojtyla arrived, +Lefebvre and others hoped that perhaps Montini was a one-off and that things would return to more normalcy under Wojtyla.  They realized after a few years that this was a misplaced hope.

Not so! If the SSPX were a congregation of common life without vows as Msgr. Lefebvre argued, the Society is governed by its constitutions and the juridical structures there established. A superior general is not a dictator; he works with other officers in managing according to the constitutions, the general chapters, and with the consultation of general council.

If, however, the SSPX were mere a pious union (association of the faithful under the 83 Code), then...bah...everyone can do whaterever they wish within the bounds of civil, ecclesiastical, and moral law. I believe it was Fr. Cekada making the pious union argument who compared the SSPX to the Sacred Heart Auto League.

Nevertheless, it remains, the SSPX was not something for Msgr. Lefebvre to do with as he pleased.

I believe it was Fr. Cekada making the pious union argument who compared the SSPX to the Sacred Heart Auto League.

My point is that the specific "accident" of sedevacantism hasn't added one iota of benefit to the plain vanilla Traditional Movement. We could list good fruits all day long from the Traditional Movement (not counting EXCEPTIONS due to bad priests -- bad men -- human beings -- sin is present wherever there are human beings).

Souls are in heaven due to the Traditional Movement. Families exist. Children have been born. Countless good.

But none of that was from throwing darts at a picture of the Pope, calling Pope John Paul II "Wojtyla", or whatever the formal component of Sedevacantism entails, practically speaking. What IS the practical expression of Sedevacantism?

How would a "Sedevacantist Traditional Catholic" date look different from a "Traditional Catholic" date? That's why I keep using the imagery of throwing darts at a picture of the Pope. It's ridiculous, but so is sedevacantism if you think about it. Not the theological position per se, but making it an identity.

Sedevacantism shouldn't affect your primary identity, your personal pronoun, your choice of where to attend Mass (or where to AVOID Mass) or anything else.

But some priests have made the dubious/unlikely theological opinion of sedevacantism into a shibboleth, a sine qua non, a litmus test of Catholicity -- and an excuse to be home alone when there are plenty of good Masses available within a short driving distance. It's criminal.

According to Fr. Chazal, Francis is a heretic and must be avoided. 

We also know that heretics and apostates can not be pontiffs - along with women, the unbaptized, the insane and those under the age of reason. 

Why is sedevacantism a shibboleth if it's merely following Catholic principles?

But some priests have made the dubious/unlikely theological opinion of sedevacantism into a shibboleth...

Because it is not following Catholic principles. A theological opinion, perhaps.

prin·ci·ple
noun

• 1.
  a fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning

Francis is a heretic.  Heretics are barred from the papacy.  You may disagree that Francis isn't a heretic, and that could be an opinion, but to say it doesn't follow principles?  Yawn...