I turn OFF Adaptive Brightness so that I can control how bright I want my screen rather than leaving brightness up to what the phone thinks is bright enough. The default option of Adaptive Brightness is never bright enough for what I want. Adaptive Brightness is the first thing I turn OFF on a new phone.
If I set me brightness to 100% it is bright enough to be uncomfortable to my eyes. With both my 2 XL and 3 XL set to 100% brightness, my 3 XL is quite a bit brighter.