Reportedly there are so many of them in Europe that it's impossible to track anymore ... and the concern is that the US may end up in the same predicament. Even if your chapel is served by a priest who's legitimately SSPX-ordained, if you happen to miss a week, now do you have to inquire about who had been there last week to decide if you can even receive Holy Communion on Sunday?
At least we can start one by one in the US as this occurs. There is only a finite number of priests in the SSPX.
Missing a Sunday and not knowing would be unfortunate, but there's only so much we can do. That would be a negative doubt, unless you asked who was there last week, if they even introduced themself.
By continuing to keep track of doubtful priests, awareness of the issue will spread and apply persistent pressure.
Of course, the whole issue is a matter of doubt, but I wonder how many NO priests aren't really priests considering the new rite hasn't been explicitly condemned by the church, and how can there be perpetual successors of Peter if those NO Popes are not priests?