The iPadOS display Reference Mode is a large part of what I need for a colour measurement app I have written. I want to disable the Auto Brightness, True Tone, and other tools that vary the display brightness, and produce a given, known brightness. I can ask the user to disable all these, but I cannot test whether they are disabled within the app.
I have found that my iPhone display is close enough to sRGB that the differences when matching to my macBook are probably due to the difference in spectrum between OLED and LCD other than any errors in calibration. It does not matter to me if we cannot set my iPhone display values to the precision or the brightness range of the iPad Pro. We expect differences from metamerism when matching different spectra. The important thing for me is to get the display to the same brightness and white in some repeatable way.