I just got a 50TU7100 and am really annoyed that all HDR inputs are way too dark to watch. I have set up the TV correctly but HDR is just unwatchable, way, way too dark. The only way I could get my 4K AppleTV to work with a better picture was to set it to SDR mode, then the image is better but still on the dark side. My TV is set to max brightness 50 but it's still not bright enough for those darker shots. In some movies like Greyhound with HDR on I could hardly see what was going on.
My previous TV was an LG which worked perfectly with the same 4K AppleTV and Greyhound played great. I regret changing TV's now, as I know Samsung will never really address this issue for existing customers, an issue that many people seem to be having for a while now.
FIRMWARE FIX IS NEEDED!
Same issue on TU8000. On TU series the Brightness setting controls the backlight LEDs, so it is useless.
I fixed it by returning the TV. 2019 RU series don't have this issue.