There have been instances that Rome has found marriages done in SSPX chapels both invalid and illicit in Annulment cases. I would suppose that this would be a major issue that the SSPX superiors would push in favor of an agreement.
Good heavens, under what grounds?
Defect of Form - lack of Catholic minister. The Case was originally an SSPV one but the principle has been extrapolated to all Trads. As Benedict said in his letter after lifting the excommunications of the 4 bishops: "They (the SSPX) have no canonical mission from the Catholic Church" thus they cannot posit acts that require jurisdiction: confession, confirmation and marriage - this is their argumentation not mine - I'm just 'splainin' !
True and False.
The SSPV case was an interesting one where a couple went directly to Rome to ask for the annulment. Rome conducted an investigation a decade ago and concluded that a number of the episcopal ordinations of Archbishop Thuc during the last 2 decades of his life were of questionable validity or outright invalid. This mainly has to do with a lack of necessary proof and the mental health condition of Archbishop Thuc during the last decade of his life (See Footnote 1). For this reason, Rome doesn't believe in the validity of the ordinations of [some] SSPV and Sede bishops. For this reason, the tribunals of most dioceses grant annulments to these couples based on a defect of form - lack of Catholic minister. I believe recently (3-5 years ago) a Sede priest appealed to Rome for a dispensation of orders or something like that. He wanted to get married or something, don't know the full details. Rome said that he was never a priest to begin with due to the questionable validity of the episcopal ordination of his consecrator. (Remember: Annulment cases go to the Diocese while dispensation of sacred orders go to Rome, per cannon law.)
For the SSPX cases of annulment, most diocesan tribunals will attribute the lack of validity NOT BECAUSE OF THE LACK OF A CATHOLIC MINISTER, but because, from their perspective and Roman declarations, the SSPX priests lack the jurisdiction or, the technical term, faculties to bare witness to the marriage. [This is why Pope Francis granted a temporary jurisdiction to SSPX priests to hear confessions, because Rome doesn't believe in the SSPX's "emergency clause" where they believe the emergency situation of the church grants them emergency jurisdiction/faculties to hear marriages and grant absolution, is valid.] There HAVE been instances (in the US at least, not sure about Europe/Latin America) where a couple have gone to the Diocesan ordinary and asked for a "dispensation" on that rule as a matter of prudence to "make sure" that their marriage is valid in an SSPX chapel and registered as a valid marriage in the Diocese. Most dioceses do so also when/if a couple is going to be married at an Orthodox Church. They get dispensed according to Cannon 1121.
But if a couple that does not have this dispensation appeals to the Diocese for an annulment, most of the time they are unfortunately granted the annulment because their marriage didn't have the proper form.
I would suppose that this would be a major reason why the Superiors would push for a formal agreement with Rome in order to avoid such problems/headaches.
Footnote 1: After the death of Archbishop Thuc, dozens of people came forward claiming to be ordained a bishop by +Thuc, some were sede some claimed to be have been sede, it was a mess. You can read all about it online and see some of the counterfeit certificates and pictures that people came up with. For this reason, Rome intervened and declared that, due to a lack of proper evidence and form and the questionable mental health of the Archbishop, many of the "ordinations" were probably invalid.