For nearly the past month I have had a Samsung G8 (last year's 3440 x 1440 version) which I bought on sale to replace the 34GP83A-B Nano IPS I had been using for two years. My IPS is actually pretty good, save for the usual issues with contrast compared with OLED. I'm lucky to have a 30 day return period, because I have used nearly all of it trying to get used to the OLED.
Pretty much right away I felt strained and uncomfortable with the G8. It was like I was always struggling to see details and I was endlessly messing with the settings to to try to get a better image. By the end I had it set to a Warm color profile and other inaccurate settings in an effort to reduce strain. When I did a final comparison between the two monitors it was clear the G8 wasn't working out.
So here is my wisdom regarding QD-OLED and why it causes eyestrain, at least in my case: It doesn't have the same edge definition with images, video and text as the IPS. I gather this is due to the pixel structure. It doesn't bother everyone but I always felt stressed using it.
I have an OLED (C1) TV and it's fantastic. but I think it works for me because I am sitting far enough away that my eyes cannot focus on the pixels. Sitting close to a computer monitor is a different story.
Next year when some of these extremely expensive 4K displays come down in price I may try OLED again. I'm fortunate that my IPS is really good, or i would be heartbroken.