Sally Jane Black’s review published on Letterboxd:
Having seen everything up to season 10 (and as of this writing, I am working on that), I have to admit, while I enjoy the mythology episodes, the stand alones are always my favorite. So this film being a stand alone didn't really bother me - especially since I know the revival series exists. Had this been the cap to the series, I might find it a frustrating ending, but as just an extended stand alone episode, it's fine. It still seems strange that they decided to make this a stand alone, though.
It's great to see Duchovny and Anderson back together after the last few seasons of the show, of course, but I was never invested in the romance subplot. Their friendship was always more endearing to me, but as I watched them together as a couple, I did see that same charm and care come through. The will they won't they plot being dragged out here didn't really work for me, even if it did create some very powerful emotional moments - those worked in spite of my disinterest.
As far as stand alones go, the execution was a little off. The editing was disjointed and confusing at times; the pacing was off; and it didn't really explain why our antagonists were doing what they were doing (the results of their work were evident, but why in the world would they want to do it? What was that man to them?). But goddamn, the horror was great - two-headed dogs, surgical horror, and at least one protagonist death that was ruthless and intense. This show has some great kills in it. It does intensity well.
My biggest complaint is the discussions of pedophilia, which at one point is argued to be something other than an abuse of power. That's unacceptable; what the film seems to want to be portraying is the potential for forgiveness for the worst of us, mixed with an exploration of faith and how it interplays with the bad in the world. That would be fine, but it wasn't handled very well. The story of Scully's faith deserves a deeper explanation than it got here, and the idea of spiritual abuse and loss of faith around the pedophilia scandals in the church deserve the same. Uniting them would be fine, but let that be the entire focus of the work if that's what you're going to do. The hamfisted moment when they talk about the connections and so on are just corny; the subject needs something more insightful.