The SSPX made a marriage agreement with Rome requiring them to seek permission from the local Modernists to have marriages.
Not all SSPX priests follow this, but it is the rule. Your SSPX priest will confirm this.
This is unacceptable to Catholic ears and Archbishop Lefebvre said there should be no co-operating with them until they have rejected Vatican II.
Then they also approved the new code of canon law (Which gives communion to protestants!). As a body of law.
They dropped their doctrinal agreement (conversion in otherwords) With Rome from their regulations. It had been enshrined for years.
They also changed their position on conditional confirmations.
Mainly though it is because of the marriage agreement (and the other ones which are harder to show), which mean we should not go anywhere near them. As a matter of principle.The supreme rule is protecting our faith, not treating the Sacraments like a childhood teddy bear we need to cling onto. Because if Sacraments were all that is needed we could just go the the Orthodox also. Valid Sacraments there, but we are forbidden because of the danger to our Faith and what it stands for by attending.
The SSPX are absolutely toxic.
By all means visit the Church and pray the rosary. It will be a Church in better shape than your local novus ordo.
But there are resistance Masses all over the US.Not all of them are listed on Cathinfo. Send me a DM. I'll tell you where your nearest one is.
And if its far, then make it a matter to go at least once a month or two months, or whenever you urgently need confession.
God will reward those who are faithful, rather than those who make compromises for an easy life.
Stay clear also of Father Hewko or "Bishop" Pfeiffer. They are Bishopless and will go nowhere in the long run.