Most people I know who are familiar with the Trad Catholic scene hold the opinion that sedevacantists are nuts. I’m not sure that this is necessarily fair. Of course the ones encountered on the internet are often far fetched in their opinions and some of the groups are cult like. But I don’t know what the majority are like. There aren’t that many of them and I have only ever met one. She was quite sane and pleasant as well. I can understand how one could end up a sede.
Basically the Sedes hold (as do the “neoconservative Catholics”) that a pope cannot be a heretic. They feel there is evidence that several or all of the last few popes have been heretics, therefore they must not be true popes. It’s really very logical.
Leaving aside the question of whether any recent popes have done things that suggest heresy. It is my understanding that popes can personally be heretics, in such a case the chrism of infallibility, and indefectibility of the church, prevents them from making their heresy the teaching of the church. I may very well be wrong about the question. As far as I am aware this is something the church has not settled. If anyone knows of clear Church teaching on this question please let me know, I have not yet read the documents of Vatican I.