While I can't compare the performance of PQ with the older Apple TV 4K, I can compare it against a few different models of Nvidia Shields and Roku Ultras. I just received my Apple TV 4K this past weekend, and thus far, the video quality is completely inline with the Shield. In general, I've always thought quite highly of the Apple TV's rendering comparatively to other options on the market at time of launch (I own the Apple TV 1, 2, and until this weekend, actually still used the 3rd-gen model for AirPlay). From a pure, non-processed image perspective, the new Apple TV 4K seems to hold up well against the competition. The Shield's ability to do some AI-based post-processing however can help it achieve subjectively sharper images, without appearing to be overly processed, in some situations.
I have noticed some slight differences to color representation, but that is because the Apple TV 4K puts out a constant HDR signal (in this case, I'm using Dolby Vision), whereas the Shield only utilizes HDR when the content being played supports it. I wouldn't call the differences in color night-and-day; I have my OLED calibrated for HDR10 and SDR, but not DV (which is nearly impossible to do on my model). This is not unique to the new Apple TV 4K however. In terms of other decode performance, I haven't seen anything that immediately pops out wrt major quality issues such as poor deinterlacing. But I don't think I have watched any sports yet on the new Apple TV 4K, which is presumably where issues such as deinterlacing artifacts would have a more pronounced effect if present.
Where have you seen posts about the new Apple TV's video quality? I searched on these forums, but couldn't find much related to that.