Indirectly related, but one thing I noticed while looking into something else was that some consecrating bishops completely change the form from even what is called for in the NO. The surrounding prayers indicate the function of the priesthood, etc, but there was one example I found where the actual form itself is completely vague and doesn't really mean anything in particular, it was just 2 sentences along the lines of "fill the heart you created."
If you watch the video, the words used are actually not that vague and arguably more relevant than that of the original Episcopal prayer. Yet, it was admitted, there are those who could ad lib and leave it up to doubt. But from what I'm seeing it isn't as vague as sedevacantists make it out to be. And the priest even mentioned it wasn't that off the mark from Eastern Episcopal consecration prayers.
This leads me to something else: the argument from intention. I've heard that they may not be valid not only because of the formula used, but also because the bishop may not intend to consecrate for the Catholic Church but the Conciliar Church, therefore invalidating the rite that way. Yet, if that's the case, why would we then turn around and accept Eastern Orthodox Orders as valid? The surely do not intend to be part of the Catholic Church, as in union with Rome, yet their Orders are not brought into question. Yet the NO would be questioned for the same erroneous intent.
A side note I want to add is that while one can conclude that NO Orders are valid, that doesn't make them licit, as the SSPX contend. Much like you could have a valid NOM or EO liturgy which would be illicit. Secondly, to properly tackle this you have to look at it from the perspective of either the Popes being true or false. If you start from the presupposition they are false, then it raises more problems about the implementation of the new rite. Yet, if they are true Popes, then there really isn't much reason to reject the changes as that falls into the scope of a Pope's authority. At that point its all speculation on intent.