Sedevacantists seem to hold no unity of practice, or even unity of doctrine.
There are plenty of "branch points" or points of division in the Sedevacantist camp.
The single issue, "Is the man Jorge Bergoglio the current Pope?" is almost tiny compared to the doctrinal issues at stake.
http://www.cathinfo.com/catholic.php/So-You-Decided-To-Become-A-SedevacantistHere's what I don't like about Sedevacantism:
1. A man making it to Mass every 6-8 weeks instead of attending an SSPX Mass 20 minutes away, because of a mistaken belief in the "Una cuм" prohibition pushed by a mere handful of extreme-position-holding Sede priests. Some sedevacantists (who I know well on CathInfo) would gush about how good it is to have doctrinal purity, "the FULL package of the truth", etc. but let me tell you: This man's life is all kinds of disorder, and his spiritual life is in shambles. He would be much better off attending a good SSPX chapel, hands-down.
2. A man nodding politely or acting like he doesn't care every time I bring up any news of the Resistance, or anything about a Resistance priest. Nevermind that we're talking about the true heirs of the SSPX, one of the largest forces in the Traditional movement! But I've met sedevacantists who act like I'm talking about some pastors in a Lutheran church! They must, deep-down, consider the SSPX (and even the Resistance!) to be non-Catholic.
3. Dogmatic sedevacantists. Need I say more? Anyone who makes a vacant chair of Peter into a dogma needs to have his head examined. The Church never declared the Vacant See to be a dogma of the Faith. To consider sedevacantism as a prerequisite for membership in the Catholic Church is ludicrous, crazy, insane, and ridiculous. And preposterous.
4. Conclavists. These are the ones who take Sedevacantism to its LOGICAL conclusion. Don't have a pope? We need to elect one! Enter Pope Michael and all the other candidates for the true Papal Throne. Who can argue against them? Once you've concluded there's no Pope, you add some simple truths to that (the Church is a visible organization; the Church must have a head, etc.) you reach the conclusion to hold a conclave.
People criticize the (Lefebvre) SSPX position as being "on the fence" or full of contradictions -- but what about most sedevacantists on CathInfo? They reject the pope, but for reasons of human respect, not being embarrassed, etc. they reject any attempts to rectify that problem by holding a Papal election.
They're holding to "Sedevacantism Lite(tm)" which is much less embarrassing before your Catholic brothers.
If you deny the Pope is the pope, but you are against electing a new one: Why? How is that any different from Recognize & Resist? You seem to be leaving a light on for the Pope, as it were. You're keeping a place at the table for him. You're still wearing your wedding ring years after the funeral.
If he's "gone", why not replace him?