From several SSPX laity I have heard this term "nutcase" being applied to Sedevacantists.

The reasons are obvious:
1 They see Sedevacantism as a threat, not to the "church" ( or "Conciliar church"), but to the SSPX,
because it is the next step after choosing to disobey a reigning pope.

2 They see Sedevacanists as being unrealistic, i.e. delusional, because they think that sedevacantists distance themselves from modern society ( by taking their religion Traditional Catholicism to the furthest extreme) that they now need to be condemned by modern society, and condemned using the morality enforcement police of that society i.e psychiatrists.

Hence the term "nutcases" applied to sedevacantists.

This argument does not convince me to give up sedevacantism, or to support the SSPX position, or even the so called "Resistance".

One might highlight the hypocrisy of the SSPX position which says on the one hand that:
1 Francis is a valid pope
and

2 We have a God given right to disobey the head of the church on matters of:
A Faith ( Re-interpretation of Catholic tradition with Vatican 2)
B Morals ( Re-interpretation of the significance of traditionally held morals in the church )
C Discipline ( The saying of the Novus Ordo mass, The new forms or matter of Sacrments, the new intention in sacraments from the new interpretation of faith, etc)

Frankly, the SSPX does not have a leg to stand on theologically.
None of the problems with the legitimacy of persevering in traditional Catholicism of the pre-Vatican 2 times occourr when the "conciliar church" is considered to NOT be the Catholic church, and the Conciliar popes NOT considered as Valid Popes.

This is why people leave the SSPX once they start to become interested in theological justification for the SSPX position because:

1 IF Francis is a valid pope, one must, under pain of Schism obey Him in matters of faith morals and discipline

2 IF Paul the 6th was a valid pope, one must, under pain of Schism, consider that all new ordinations and consecrations are valid without the slightest doubt, and all the new methods of administering the sacraments are valid because the church authority decided they were.
AND
 The new mass is valid

3 The arguments against Sedevacantism such as:
"Sedevacantists are crazy"
"Sedevacantists are protestants"
"Sedevacantists are not using their brain"
"Sedevacantists are heretics because they say that when one is at an SSPX mass that because the priest is in communion with the pope that means that he holds that the pope has the same faith as him, and is therefore praying that he continue in that faith, which is anti-traditional since vatican 2."

Are JUST NOT GOOD ENOUGH.

The arguments are not good at all in fact. In fact they are VILE and diabolical.

I maintain nonetheless that anyone who willingly blinds himself to the truth on any matter of faith or morals does so because they are more comfortable denying a truth. They love their comfort more than the harsh truth, and are sinning by omission by doing so.