I sometimes run an ultrawide (3440x1440) off my M1 Air. It works... ok. MacOS doesn't output more than 50hz, and every hour or so the display will flicker off and on. I haven't decided if its an issue with the M1, or an issue with the hub I'm using, but its not a big enough concern for me to justify investigating much further.

I personally feel that 21:9 ultrawides aren't quite as good as dual 16:9 displays for productivity. But they're fine. My biggest complaint is that most people would reasonable center their monitor on the desk. If you run 2x16:9, you can have one centered, and one off to the side, which allows you to mostly look straight forward and have a natural primary/secondary setup.

At 21:9, this setup is weirder; either you size your primary window (browser, code editor, etc) to ~16:9, left-or-right-align it, then you have to slightly crane your neck, or you center-align it and you're left with slivers of screen real-estate to the left-and-right. Or you can shrink the primary center window, approaching maybe a max of 7:9 for all three windows, which is maybe rarely useful for some applications. Or you can maximize the window, and in most applications you get tons of unused space. Or you can physically offset the monitor on your desk, which looks weird. There's just no solution that works "great" especially given these displays are oftentimes at a small premium over 2x16:9.

I think something like the Samsung Odyssey CRG9 would be much higher productivity in ultrawide, with its 32:9 aspect ratio. There, you could run a ~16:9 centered primary window, and still have ~8:9 canvases to the left and right, which is enough horizontal estate to comfortably have a terminal, Spotify, browser, etc. Its also relatively affordable (~$1100), though given you can find solid 1440p 27" monitors for $200-$300, you're effectively cross-shopping 3x16:9 displays. Still definitely a "luxury" buy. Its ultimately unclear to me that it would be better, and not just "differently similar".