If I look at my Xs MAX straight on, looks good to me. The moment I tilt the phone up or down, I see one part brighter/different than the other. I figure this is just due to perspective because I am tilting the phone (up/down/right/left). Other people that have told this to me, I usually look at their phones straight on (or close to) and it looks fine. The moment I look at a different angle, yeah, the screen will be a little different.
Even my MBP ... if I tilt my head to the right by a foot, the right part of the screen is easily 2x brighter than the left. But straight on, it's fine.
It is depending a lot of quality of the panel. From what I have read it has something to do with from what part of the "big panel" it was cut. These panels are manufactured from one large sized panel and then cut to current size and those further away from the center have more or less problems like you described. My first X had awful screen where lower and upper part of the panel had different hue. My second one is acceptable with no obvious difference in hue but it is still pretty much depending on viewing angle so it must be viewed pretty straight or in very slight angle. Another X we have in our family has one of the rare good panels with much wider viewing angle very even in all parts of the screen. It is pretty much perfect panel with brighter material and I'm very critical viewer usually seeing even the slightest problems in displays. With darker near black levels situations is again different but that is more or less inherent to OLED.