It becomes frustrating when the activity stack is invisible, but still accumulating additional activities in app. Old Gmail was especially egregious with this,

It becomes frustrating when the activity stack is invisible, but still accumulating additional activities in app. Old Gmail was especially egregious with this, where you could hit the back button 10+ times and end up switching between emails you've viewed days previously before you actually back out of the app. I love the general idea of the separate Up and Back, but I'd compare it to XP-era Windows explorer rather than a browser (though both are valid comparisons, I just think Explorer is closer). It just runs into issues where the developers have to try to divine what exactly you are wanting to do at each instance when you hit back. Sometimes users want to go back to where they were before when they hit back, sometimes they want to go 'back' to the home screen of an app - this is what the 'Up' navigation button should exist as, but seldom does. What about if you go Up in app, then hit the back button? Should you exit out of the app? Go back to where you were in app? Go back to the previous app? All are valid cases. It really does all come down to consistency.