I spent significant time with the OLED PG27UCDM at the ASUS booth at CES this week. When you set the display to the DisplayHDR 400 True Black mode, the accuracy is very good with HDR mode enabled in Windows. Quite impressive for a gaming monitor.
However (with the firmware on this unreleased demo model), all other modes show very poor accuracy. This includes all DolbyVision modes and the Gaming / Cinema / Console HDR modes. Photographs viewed in a browser (Edge) looked terrible. Contrast of standard images isvery low, colors are very inaccurate, HDR highlights are clipped, etc.
The hardware is clearly very capable and impressive. This just seems like an issue that can be resolved in firmware. Hopefully this is resolved before launch, but if you get this display and the image quality looks low, just go to the OSD menu / Image / HDR setting and set it to DisplayHDR 400 True Black mode. The results in that mode are great. I would be very happy with those results - and I would be thrilled if the 1000 nit modes were improved to a reasonable level of accuracy (not that I expect they'll be as good as what is achieved in a limited mode).