Talking about android. It's never quite clear where the back button will take you (sometimes it goes back to the previous app, sometimes not), some apps (in particular the google app!) lose their state on switch, the switch process itself is fiddly.

   It's never quite clear where the back button will take you

I have an Android phone where the back button is drawn on the screen, not a hardware button. Why can't it display some additional information about where it goes when I tap it?

It can be quirky for sure, but I personally got used to the idiosyncrasies pretty quickly.