Counterpoint: The way that every single screen on iOS has a separate "back" button you need to look for vs just having it unified by the OS is an incredibly frustrating UX.
I'm not sure why someone downvoted this. Every time I use a friend's iPhone I find this frustrating. Like anything else it's something you get used to either way, but "back" is such a common action that it makes sense to have a unified way to do it.