I cannot think of one game that was not playable at 21:9, some games you need to take 1 second to click on "flawlesswidescreengaming" fix, some games you need to spend 10 minutes finding a fix for either the HUD or cutscenes, some games work perfectly no problems, overall every game works, most games with no problems, some games that have HUD at 16:9 (but overall still better than 16:9) only game I can think of that does not work at 21:9 is the forza beta, but that is microsoft store which only a few days ago supported above 60fps......
Also Max payne and witcher 3 cut scenes but again there was a fix... I admit I do not like downloading fixes, but most of the time you don't need them... or is a simple config tweak so no download needed, overall I would not want another 16:9, maybe a 32" 16:9 but nothing lower than that, big 16:9 monitors over 32" are too tall so 21:9 34" is the perfect size for a monitor, maybe 35" or 36", I have also used 24" 16:9 27" 16:9 32" 16:9, also 3440x1440 34" is the best combination of good image quality and still being able to get ok FPS.
The monitor to end all monitors I think would be OLED 35" 3440x1440 DP 1.4 200hz Gsync, maybe in 5 years that will be available.