Setup:
Laptop with 2K screen and a fullhd screen (1920x1080) with displayPort

Go to systemsettings > display config
and set 175% scale for 2K laptop screen, 100% for fullhd screen.
This makes apps/windows on both monitors the same visual size.

Problems:
- re-open  systemsettings > display config and try to rearrange the screen-miniatures.
They 'jump' around, you can't position them properly with the mouse (until you reset both zoom values back to 100%, then it works as expected)

(reapply the 100/175% setup)

- open an image folder (e.g.  /usr/share/wallpapers/Next/contents/images/) and select the 1920x1080 image size picture. Right click and open gwenview on the full-hd screen
The image is shown, but scaled - even if you set gwenview to 100% or full-screen.
As the monitor has 1920x1080 pixels and the image show has the same pixel size, i would expect it to be filling/covering  the whole screen, but gwenview (maximized) only renders a smaller version of it

- now, click on home button in gwenview to show the folder's content
The preview icons are not rendered correctly, stacked over each other and if you move the mouse, the icons become unusable.
This can be fixed by using the slider (zoom in lower right corner of gwenview) and moving it back and forth to the same view it had initially. Then the icons are displayed correctly.

- in dolphin, open the /usr/share/wallpapers/Next/contents/images/ dir again and press F11 to show the (initially hidden) image preview. This destroys the whole dolphin window contents (making it unusable because of missing buttons/list-panels or symbols) until you close/reopen dolphin. Then it works ok.

- i also have the impression, the the font renderings on the fullhd monitor are not as smooth (appearing pixelated) as compared to normal (100/100% scale), this is especially true for small fonts (like thunderbird, a browser like chrome/firefox and typical webpages render smoothly)

- the mouse scroll-wheel behaviour on browsers also 'jumps' when being operated slowly. The impression is that with each slow wheel movement, the browser scrolls up/down 3 or 4 lines at once (giving this jumping look)