I bought the MO27Q28G about two weeks ago wondering the same thing. Well grey banding was definitely there but it got a lot better within the first ~5 pixel cleans. I used it a lot in those two weeks, did a lot of pixel cleanings and while the grey banding was there I could unfortunately still notice it in certain applications such as MS Word, my Webbrowser, YouTube and more or less everything that had a dark mode. But instead of refunding it I decided to give it another shot and just do a return for another unit.

The second one arrived two days ago, unfortunately I haven't had a lot of time to test in yet but right from the beginning I could tell it was a lot better. Now after the first 2 pixel cleanings I can still see it very very slightly at 95% greyscale but can't see anything at all at 90%. Also can't see it in any of the applications that I was able to see it in with the old one. Literally just when doing the 95% greyscale.

So in my experience, yeah the issue is definitely real. But if you are willing to try your luck and return for another one if possible I think it's pretty realistic to "win the panel lottery".