In looking at the photo of your screen, if the photo is a good representation, it looks like the left side of the screen is slightly brighter than the right side, but not by much. It also looks like you have a stuck pixel in the lower right hand corner. Personally, I don't think the brightness difference is really anything to be concerned with as you have to set the brightness very low to see this. I would be more concerned with the stuck pixel that's on all of the time as when I see those, I can't unsee them and wind up looking for it all of the time. On a generally light or white background, you won't really notice it, but on a dark background, it jumps out at you.
I read through the article that @braintheboss linked and I don't see any of those types of artifacts on my screen at all. It is very clear and sharp and consistent in brightness across the screen. I'm not sure how similar the panels are between the Dell screen and the ASUS screen as our screen is a touch screen where the Dell is not, so at least some of the specifications are different between the two.
In the end, it's up to you as to whether or not what you see on your screen is acceptable, but with what you've shown me, I think you're well within the normal range of variation of the screens and I wouldn't be worried about it
Good luck and let us know if you will keep the laptop or return it.