Neven Mrgan’s review published on Letterboxd:
It's perfectly fine for a movie to have an elusive core, to not be about anything in particular. But when that is the case, it should at least populate the screen with enough varied ideas to invite engagement. The Master is a claustrophobic, one-track tale(?) and so the overwhelming emotions it invokes are befuddlement and, often, boredom.
The titular Master is very mildly based on Scientology's L. Ron Hubbard, and references that spell this out do the movie no favors, turning it for brief seconds into something close to parody.
Phoenix's performance and the cinematography are two examples of true mastery here. Hoffmann does great with what he's given, but that may not, in the end, be very much. Many of his soliloquies here open strongly and end stronger, but in the meantime, nothing of import seems to be said. As the movie nears its climax, we're treated to a scene where a trusting follower of The Master's discovers the frivolity of his teachings and the explosive nature of his temper. Powerful stuff, except it comes as absolutely no surprise to the viewer who has both suspected it and seen it played out by that point already. Nor should it surprise the follower, who, we saw, has had ample opportunities to witness the same behavior previously.
Similarly, Phoenix's character is compelling, but only to a degree. Once it's clear that he's not on a journey of any sort, that he learns nothing, discovers nothing, doesn't build on his experiences at all, the viewer may feel even a bit of relief: ok, he is who he is. Unfortunately this is also when the movie decides to end. And so what to make of it all, then?
In a recent interview, P.T. Anderson says the movie was basically assembled in the cutting room out of a lot of scene-based footage. Many great movies got made this way (Annie Hall comes to mind) but boy does this one show the seams where it was sewn together.