I don't know mich about windows GUI, but from my user experience I think it's just hard (lots of things to consider).
I guess you have to deal with font sizes (handled by the system) you're own contraints ("this button is half this screen", "this screen is at most 800px" etc), and the user scaling your UI.
Before scaling was even an issue, I've seen a lot of app screwing the font and UI size pairing when launched on non english languages with different default fonts settings or wildly longer text(the app's own translated text that is).
Throwing in a "what dpi is my screen?" variable to the equation must make things that much harder.
I guess you have to deal with font sizes (handled by the system) you're own contraints ("this button is half this screen", "this screen is at most 800px" etc), and the user scaling your UI.
Before scaling was even an issue, I've seen a lot of app screwing the font and UI size pairing when launched on non english languages with different default fonts settings or wildly longer text(the app's own translated text that is). Throwing in a "what dpi is my screen?" variable to the equation must make things that much harder.