Macs only look good at a 110 or 220 pixel density.

34" ultra wide 1440p has a pixel density of about 110 ppi, and that should work just fine. You might need to use a different cable, though. You should look your monitor up on rtings.com and search for notes on Mac compatibility.

I have a MacBook Pro 14 that I use daily with a 34" ultra-wide 1440p gaming monitor, and I've never had a problem. I had a MacBook Pro 13 before the 14 that also worked without issue.