200 nits is totally fine for SDR content in my experience. My Samsung G70A LCD is not the brightest, maxing out at around 400 nits but I run it at about 120 nits, day or night. I used the LG CX 48" at the same brightness level. For me this is totally ok because I can control for light in the room. It does not mean I use it in a dark cave, just that there is no direct sunlight hitting the screen or high brightness overhead lamps.This thing is laughably only 200 nits bright. LG OLED is terrible.
200 nits is also what the PG42UQ has for it's "uniform brightness mode" so it's most likely how they avoid ABL in SDR content.
Hopefully in HDR it can perform similarly to the 42" OLEDs because that's where you want the higher brightness.