Although the promised contrast ratio 1000:1 was not achieved (BTW nobody expected that) 819:1 is an excellent result for IPS. It’s slightly better than the reference monitor shows – up to 800:1 (same in it’s specification).
But the “quality” of 2490 contrast ratio is better as it corresponds to lower black level.

Brightness regulation
The NEC 3090WQXi has a new mode of brightness regulation. Let’s look at brightness control scale. Backlight intensity reduces brightness from 100% down to 73.2%. Below that point brightness is digitally compensated as manual says. It makes the depth of black stay at 0.3 cd/m2 with no change beginning from 230-240 cd/m2 of white, while brightness of white gradually goes down to ~62 cd/m2. So contrast ratio gradually reduces as well. I am not ready to say if this new system is good or bad. I guess, NEC’s engineers designed brightness regulation this way for a particular purpose.
The reference monitor NEC 2490WUXi has “regular” scheme of brightness regulation: three independent levels. The first level is managed by backlight intensity only and covers ~230-240 to ~ 130-140 cd/m2. Low Brightness mode decreases (by backlight and “digitally”) brightness by 50%, and so does Advanced Low Brightness mode – by another 50%.