As for going back to the previous app, as I argue below, the back button is crippled there too because you need to be in the root view controller of whatever stack you're in for it to do that, and you need to remember which app sent you there, which is not indicated in the UI.
In iOS you can get back to the previous app from anywhere in the navigation stack with a double-tap gesture. An additional tap, to be sure, but all ambiguity from the constant and ever-present back button is removed.