I would add this thought --
Lack of noxious doctrine is a good start, and acceptable for THE SHORT TERM. However, lack of positive good can cause problems long term.
Here is an example of what I mean: in the Novus Ordo, various changes were made to the Liturgy and teaching practices, which are 99% sins of omission. No longer calling the Mass a Sacrifice: just saying "meal meal meal" 9 times out of 10 which is true, strictly speaking, but is only part of the truth. By exaggerating one aspect of the Mass to the exclusion of the others, it's a distortion and ultimately a lie. Because, in effect, they teach by their OMISSIONS that the Mass is ONLY a meal, which is FALSE.
Also, the Church never came out and defined or declared that Purgatory doesn't exist. But by changing the funeral Mass, regular sermons/teaching practices, subjects and priorities, they ended up convincing the majority of Conciliar Catholics that Purgatory doesn't exist. I know of N.O. Catholics as long ago as the 1980s who literally said they don't believe in purgatory. DESPITE THE FACT THAT NOWHERE IN VATICAN II DOC'UMENTS OR QUOTES FROM CLERICS WAS PURGATORY DONE AWAY WITH.
Now apply this to modern-day SSPX priests "keeping their head down" and you'll see my point. Yes, for those trained IN THE OLD FAITH they'll be OK, perhaps for years. The omissions won't hurt them, since perhaps they learned those things elsewhere. But the next generation will be in trouble, due to what these priests FAIL to teach and preach in their sermons. When and where will they learn about the hol0 hoax? It's a replacement for Christianity. It's NOT a distraction, a nice extra, or something that doesn't matter. It matters like nothing else!