In my laptop 200% fractional scaling of the built-in QHD+ OLED display on KDE Plasma is great, but when I connect my portable 1080p displays or e-paper monitor (UXGA) the fractional scaling of it is not as good as when compared to when the same monitors are used with macOS (M4) in HiDPI mode under lower resolutions.

As the other comment says, I suspect it to do with GPU as the built-in displays are usually driven by intel and external displays are always driven by my Nvidia 4090.

But surprisingly, when I drive my e-paper monitor using original Jetson Nano (Ubuntu 20.04 ESM / Nvidia Maxwell); the fractional scaling is quite good perhaps even better than macOS.

IME it works fine on Intel iGPUs, too. I even use multiple ratios with no issue (constant 125% on the laptop, 100% on an external screen and 200% on a different screen at work).

Contrary to Windows, this also works well when hotplugging screens. Menus don't end up a blurry mess, and apps seem to work fine. On Windows, even some 1st-party apps are borked (task manager comes to mind).