"It is not so easy to prove a heresy as many people think. The Catholic Church has always dealt with this accusation with extreme care. It is the gravest accusation you can make. When you consider that this accusation is directed to a Pope, you understand that it is impossible to deal with a more serious matter. However, I am seeing some persons who lack the needed study and maturity who are publicly labeling Popes, Councils, Cardinals, etc, as heretical. There is a certain brave sede-vacantist young man, still in his twenties, who claims that he found more than 100 heresies in the present Pope's statements and more 200 in Vatican Council II. He openhandedly condemns to hellfire all those who disagree with him. The least I can say is that he is somewhat hasty in his judgments. The practical consequence is that this kind of accusation, without the needed evidence, is more harmful than beneficial to the good cause. In face of this kind of imprudent action, the serious scholars who have spent long years in hard study and who are giving their blood to defend the Catholic Church run the risk of losing all credibility if they keep such company.
Second, if some heresy were to be proved, it is not so simple to decide that the Pope lost his pontificate. As I said before, there is no consensus about "when" a heretical Pope loses the papacy. Therefore, the position of many sede-vacantists, which presupposes that when a Pope falls into heresy he immediately loses his pontificate before the Church, is not exact. It is a simplistic solution to a very delicate and complex problem."
Atilia Sinke Guimaraes Resistance Versus Sede-Vacantism