What's the issue you have with high DPI monitors? I've used 3200x1800 14" screens way back (on Fujitsu U904 when that came out: I found a review from 2014 online), 4k 24" Dell when it still required two DP cables for 60Hz, and more recently 4k 14" screens on X1 Carbon: while you need to configure scaling (I prefer 125% or 150% for UI elements, and fonts further increased by a factor of 1.4x), most programs work well with that (including non-native UI peograms like Firefox, LibreOffice or even Emacs).
For a long while there was an issue with multiple monitors which you want to configure with different settings: you couldn't.
I believe that is also fixed today with Wayland but I mostly stick to a single monitor anyway.
Programs? I meant kernel and drivers. I don’t even need to open an app. My ASUS laptop with a 4090 steadily fails with an LG 40WP95XP with anything else than 100% DPI. My previous ASUS N552VW failed quite often on kernel level because it couldn’t handle the built in 960M, and it definitely couldn’t handle at all my older ultra wide monitor (I don’t remember anymore what was the model exactly).
Please describe "failure": I've had a Sony Vaio Z with switchable Intel/Nvidia graphics in 2009 before Optimus (though that did require some tinkering), but had GTX 960 and GTX 970 (actually still do) in a couple of computers, along with an integrated Intel and AMD GPUs in a bunch of laptops.
Note that kernel is totally unconcerned with DPI in general: it only cares about physical pixels and reports physical dimensions to apps — if scaling caused kernel level issues, it might be related to proprietary driver issue (they frequently lag in Nvidia's case).
I never used ultrawides myself, but if the monitor did not report proper "timings" and available resolutions, you might have needed some manual tweaks.
For a long while there was an issue with multiple monitors which you want to configure with different settings: you couldn't.
I believe that is also fixed today with Wayland but I mostly stick to a single monitor anyway.