This is curious, I wonder how the reviewers have set up? A great many reference photo displays max out at 300-350 nits, and are typically set to 1/3 to 1/2 brightness for any work that will end up on paper. So, either Apple is inflating the spec, or... reviewer is trying to watch max brightness in a dark environment and complaining about black levels. My guess is Apple is confusing reviewers (pro and non-pro alike) because of a dissonance between the screen and the wrapper. It could be a pro spec screen if it is color accurate (minimal standard deviations), is true 10 bit (not 8bit+FRC), and reproduces 95%+ of DCI-P3 gamut, and has a quality dimming control* It's definitely a consumer/user friendly wrapper, in a good way, in all but price.
*pro-tip! *** no perceptible flicker on or off axis from the viewer's line of sight, not the screen. If you want to easily see the how good a display's dimming control is or isn't, dim it in a dark room, look at it straight on and then turn your head until and look off screen until you can just catch the screen in the corner of your eye, your peripheral vision is far more sensitive to flicker than the your central vision.
I think a lot of reviewers don't really know what to test here in part because of how Apple positions this product, and in part because they really don't know what to test since they don't edit for print, or color grade. They read text, they view content, they play games, and they don't know much about ergonomics. It's possible that this display ain't that good, but these reviews don't tell us much about how a properly set display works. For starters any display should be set to the minimum brightness possible for a given ambient.
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