On a technical level, there are still some apps that can get CoreWindows, respond to PLM events, receive callbacks from contracts involving CoreWindow UI like Share target, as well as go into certain UI modes involving shell participation like modern fullscreen and CompactOverlay, and some that can't (without stapling them together with one that can via Desktop Bridge) ... aren't there? Maybe that will change in the future but it hasn't yet.

And then there are some OS editions where some apps can't run or can only run in a container environment.