Ironically, MKBHD is one of the first reviews I saw, and they encountered the overheating warning and were not able to adjust the brightness. It also happened again when he was navigating in Google Maps. Here is the link - the issue starts right at about minute 9.
https://www.youtube.com/watch?v=7QH1qFd24bY
It's one of the videos that caused me to come here and chat with people who have the phone and get real world experience. Someone in this thread asked me for links, and I finally got a chance to go back and look through some of the video reviews I watched. There is at least one more that mentioned encountering the same issue, but I think everyone here already gets the idea.