OK, in the same vein:
If an FSSP priest was ordained according to the traditional rite, why might one still have reservations about the validity of his sacraments?
While I completely agree about #4 and have lived that conviction my whole life and I personally believe there are many invalid NO priests and bishops etc. out there, there are two other trains of thought to consider.
1) The Church, by divine right being the only safeguard, holder and defender of the sacraments in this world, has always and everywhere presumed validity of all the sacraments until or unless proven otherwise. This includes NO consecrations/ordinations, which are of course, the sacrament of Holy Orders.
To presume invalidity without first proving invalidity, is denying the divine prerogative given to the Church to defend, protect and preserve the sacraments till the end of the world. IOW, presuming automatic invalidity means the Church is not defending, protecting or preserving anything at all, not even thin air.
2) If we presume validity as does the Church, then the valid NO consecrations / ordinations are exponentially (infinitely?) more sacrilegious and damaging than if they were invalid. It is this fact, in and of itself, which gives us the right, nay the duty, to completely avoid the entire NO. I know some will not grasp this reality, hence won't understand it this way and instead will insist positive invalidity as if such a thing can even be known, but that is not how it works.
In the grand scheme of things it (validity/invalidity) is actually an academic point because as long as the NO priests and bishops etc. continue in their abominations, we can have no part of it, and this is true whether they are valid or invalid.