Got the AW Ultra yesterday from Amazon and it has the same issue. In a darker room the brightness is far too low. This is definitely not a "getting used to it"-effect, it is simply much too dark.
In a bright surrounding, the display is great and works as expected.
So I checked all settings (full brightness, sleep time, etc.), did two hardware resets, one to restore a backup from my former watch, one to set up the ultra as a new device, no change, problem remains.
I do not believe, that it is a software problem, because in a brighter surrounding the sensor works without any issues, but it fails in darker surroundings.
In addition, many ultra watches work well. This indicates that there is a hardware issue of a batch.
I talked to the Apple supprt today and we came to the conclusion, that this may be a problem of the light sensor.
So I talked to Amazon and they will send me a new watch. Great service, thumps up.
Anyway, I will check if the new watch works fine.
In general, I would be happy, if I am wrong and there is an easy solution for the issue.