His errors are myriad, but the sedevancantists make one specific error that separates them from Rome. That is the fundamental difference. It's not that sedevacantists make more theological errors than the Pope---it's that they make one particular error that on its own severs ties with the line of Peter.
True. Those who recognize the current Pope's contempt for the Deposit of Faith are justifiably scandalized. The extremeness of his behavior makes it tempting to want to escape his orbit by denying his legitimacy; however he sits on Peter's throne due to God's permissive will. We need to pray for Francis (along with his partners in chaos). "And we know that to them that love God, all things work together unto good, to such as, according to his purpose, are called to be saints." (Rom 8:28)
I would argue that the sedevacantist error (if they are even in error which I obviously would say they are not) is not one that severs them with the Church. It would be an error of fact. At best we would be guilty of making a sincere mistake as to the identity of the true pope.
I don’t think anyone can know for sure how God will judge those of us who have tried to remain faithful to the best of our ability in these unprecedented trying times.
God Bless.