There's been no small amount of problems with Windows' inability to adapt to the new wave of high-dpi displays. My partner got a Yoga 2 Pro with the 3200*1800 screen and for example the youtube player controls are almost impossible to interact with in some view modes (even with a mouse pointer). I can see this being similar, in that if the app determines at runtime how to display its controls then there seems to me like there will be issues, for example with reflowing text or image positions to accommodate button size changes.
I mean, the alternative is to have Mattel's My First Computer and have all UX elements touch-size by default but then why even have a high-resolution screen with more precise KBM inputs? I know that I don't really want huge icons on a 4K monitor because i want significant screen real estate.
I think that it's going to be interesting to see how they strike this balance.