Not all UI elements scale with font size.

Illustrations, nested element containers, navigation controls, etc. It looks very comical when you first see it.

Desktop apps scale their layout based on the number of pixels, not on text size. So (with the notable exception of browser apps and by consequence Electron apps) you will just end up having bigger text in a badly laid out window if you just increase text size.

Big fonts, small UI components. Looks bad and requires more accuracy with mouse movements.

Padding inside boxes/buttons looks all weird.

Since for me less is more I'm doing 200% non-fractional with a smaller font. The problem is that I need a different font size from my 1920x1200 laptop screen (usually secondary, far away from me, with documentation websites/slack/etc) and my primary 4k.

For browsers I can change the default font size and forget the controls, since I'm using vimium, but stuff like Skype/Slack/Zoom/etc needs more dancing whenever I change the display.

Most of the time that is actually much better than fractional scaling.

On a laptop you are severely constrained by the screen real estate, so minimizing the size of the clutter and focus on the content (text) is a great tradeoff in most circumstances.