Don't have much experience with ultrawide unfortunately.
I did notice the ASUS PG39WCDM and LG 39GS95QE-B are OLED.
You might want to check out these series of articles on OLED burn-in with heavy desktop use:
The OLED Burn-In Test: 9-Month Update
I've never had an OLED monitor, so I can't speak from experience, but the findings of the above-mentioned experiment are not very comforting considering it's a mere 9 months of running the test. I have some older OLED smartphones with burn-in and it's extremely annoying for regular use.