How do you know for certain that all FSSP priests are validly ordained? Were you at each one's ordination? Are you trained enough to know if something in the ceremony is "off"? How do you know the ordination wasn't a 'hybrid' of the old/new, with just 'smells and bells' of tradition?
Even IF the FSSP priests are valid, were they properly trained in the seminary? How can any of us know? Do they say the 1962 missal, strictly and follow all the rubrics exactly? Who taught them to do so - novus ordo priests? Ha ha! That's like saying a first year med student could teach someone how to do heart surgery. The rubrics require precision and attention to detail - something the novus ordo neither requires nor encourages.
Thirdly, which host are you receiving at mass? The one from the novus ordo, or the one from the current mass? They are all stored in the same tabernacle. The FSSP accepts the novus ordo, so would they spend any time separating the hosts from one another? Of course not. To them, each host is the same. Odds are high that you are receiving an unconsecrated piece of bread from the sacrilegious 'mass'.
Then, there is the problem of the public scandal that the priest and you are providing to those who go to that church. By attending/saying an indult mass, you implicity accept that
1) the latin mass is NOT the ordinary form of mass, but has been replaced by the novus ordo
2) that the novus ordo is valid, as a mass (highly doubtful, per Cardinal Ottaviani) and a sacrament (communion could be valid, who knows? It also could not be),
3) that the novus ordo is the same rite as the TLM (as pope benedict argued in his motu, they are "two usages of the same rite")'
4) that the novus ordo is acceptable to God, which it cannot be because it sinfully violates the legal and moral commands of Quo Primum.
The Indult is very problematic, both morally and legally. If you allow God's Divine Providence to be your guide, and you put your trust in Him, and LEAVE the indult, He will bless you. If not, you are putting yourself in harm's way, and are compromising your Faith, even slightly, which we cannot do.
As a former FSSP seminarian, I can answer all these questions with certitude.
How do you know they're not? Are you God? Have you ever seen ordinations by any of the Ecclesia Dei orders? They all use the old books, the exact same rites and the same as the SSPX and every other traditional order in the Church uses. If you think they do otherwise, you are very ignorant of how they operate and are assuming a lot. You don't have to agree with "Indult" orders and the way they were established, but stop spreading disinformation which simply isn't true.
As regards to seminary formation, since I was there, and knew pretty much every seminarian and priest, I can assure you that they taught the Catholic faith, Thomism, Catechism of the Council of Trent, the Syllabus, 6 years of Latin, etc etc etc. It is a traditional seminary, not a Novus Ordo one. I have seen probably over 30 FSSP priests celebrate mass, and not one of them has violated the rubrics from what I can tall, not even once, so please spare me your diatribe which is completely unfounded.
The FSSP doesn't share Novus Ordo churches any longer than I'm aware of. They did have a few shared parishes at one time (many years ago when they first started) but they do not any longer that I'm aware of. You are also assuming every Novus Ordo is invalid, which it isn't, even the SSPX doesn't believe that.
1. There is no "official" stance from the FSSP regarding ordinary vs extraordinary form, that was simply Benedict XVI's opinion. No one is bound to it under pain of sin. I know plenty of FSSP priests who disagree with the distinction.
2. Even the SSPX accepts the new mass can be valid. Even Fr. Hesse. You are taking it to an extreme.
3. The FSSP doesn't take any official stance on that. Pretty much every FSSP priest I've ever met doesn't make that distinction, they just refer to the Novus Ordo as the Novus Ordo and the traditional mass as the traditional mass. You should read some of Fr. Ripperger's Latin Mass Magazine articles on the subject such as The Merit of a Mass. Stop assuming things about people.
4. The FSSP takes no official stance on this that I'm aware of.
I'm not saying I agree that an Indult should even exist, but I have never once experienced my faith being compromised at the FSSP, never. You're doing souls a great injustice by telling them to avoid the traditional sacraments and good priests. God has blessed and continues to bless the FSSP, although it was born in imperfect circuмstances; it is nonetheless bearing much fruit and many graces, to say people should avoid it shows you really know nothing about the formation of priests in the FSSP and how they are formed. I would urge you to have a little more charity towards those who desire to grow in holiness in an indult parish. Not everyone has access to the SSPX, not everyone is convinced by the arguments of the SSPX. I personally do not really care either way, I think both organizations do the work of God.