If you take nothing else away from this letter, at least hear this —the SSPX’s marriages and absolutions are invalid because their priests lack the necessary faculties.
The SSPX argues for the validity of their marriages and absolutions based on the canonical principle that the Church supplies the faculty in cases of doubt or common error. In certain rare and exceptional cases that might apply to their situation, especially with regard to confession, but for the most part their arguments are not persuasive.
Part of their argument hinges on the faithful erroneously believing that the SSPX priests have the requisite faculty; well, if you were in error about that up until now, you are not in error anymore.
The SSPX also makes the argument that they have permission because the Church is in a state of “emergency.” However, 1) the Legislator (the Pope) and the bishops with him don’t think there is a state of emergency, and 2) the sacraments offered by the SSPX are already widely available at legitimate parishes and chapels, i.e., no one is being denied the sacraments.
This is not the place for a discourse on the technical points of canon law, but the point is: do you want to take that kind of a risk with your marriage or even with your soul?
Great letter to his people from Bishop Morlino about the SSPX. Here's a problem that's important and that I'd never thought of (given that I'm not much of a devotee of the Extraordinary Form, though I support people who love it having it):