I had a strange problem with Dell P2720DC (27'' 2560x1440) - the whole outer edge of the screen would flicker if I used dark background with a lot of dim colours (like dark mode in IDE, or default Grafana theme). It wouldn't happen all the time, but it would happen on a weekly basis (I most often seen that on Sunday). I've RMA'd it, got another monitor, which also started showing the same problem. I gave it to someone who doesn't use dark mode - no issues.

So, I'm not getting another Dell until I'd be sure this issue won't happen again :)