Gonna bump this as I just bought a new Xs, came from an 8.
The Xs I have has uniform color and the brightness is identical to my 8 as well as my wife's Xs, and multiple display models at the store (as well as a couple exchanges I had to do in buying mine).
I was in Best Buy today to buy printer ink so I figured I'd compare mine to the Xs/Max/Xr they had on display because why not? Mine was a little brighter than the Xs, and a LOT brighter than the Max (yes, TT/NS/Auto brightness all off). The max was actually dingy green in the upper right quadrant. I felt pretty good about mine when comparing. But, then I put it next to the Xr and it was no comparison, the Xr with its LCD was so insanely whiter than everything else it wasn't funny, crappy pixel density not withstanding.
I wouldn't say the Xs/Max have yellow tint so much as the calibration is just brutally warm on these.
But, I have to say though, I don't see how the 8/8+/X could be calibrated to 6500K, and then the Xs/Max also be calibrated to 6500K, and be clearly warmer than the 8/8+/X, it makes no sense. To make the 6500K claim even less believable, there's no way you can hold an Xs/Max next to an Xr and tell me they are all 6500K, because they simply cannot be. As an engineer, the science just doesn't add up here. You can say OLED just can't do whites like LCDs, and I'll believe that, but you just can't put an Xr next to an X/Xs/Max and tell me they are at the same color temperature, or even within panel variation tolerance of one another and be 100-150K off. No way.
I'm happy with my Xs, but holy hell can we get a white point color temp slider? Color Filters is NOT a substitute for proper white point adjustment. Accurate or not, not everyone likes a warm/accurate screen; I do no color accurate work on my phone so what do I care? I just want it to look good, and that means cool and crisp whites.