The cad tools is still correct but I've been using mixed DPI multi-monitor for a while now on plasma Wayland without any issues. I have them set to different scaling levels as well
Wayland can do mixed DPI multi-monitor setup, and Onshape is a fantastic CAD system - it runs in browser and works perfectly on Linux. I used exactly that setup profesionally for nearly 2 years.
Probably Nvidia users and devs that don't want Wayland. I have yet to encounter anything that doesn't work apart from things I only read about in arguments by devs who adamantly refuse Wayland for Reasons (TM)
Things do work on Wayland and often with less fuss. Those criticisms are often opinionated hardline views. Some have truth to them, but they throw the baby out with the bath water.