I have been using Android exclusively for almost over 1.5 years. And while I enjoy custom keyboards, it does not provide a good user experience on Android. The biggest issue is that on my Nexus 5 every time my keyboard has an update from the Play Store, it gets disabled system wide. So I have to go into the keyboard app, go through settings to re-enable the keyboard, then wait for a popup where I select the keyboard as my default, and then I can start typing. It is a huge annoyance that takes away from the user experience. And I have used Kii, Swiftkey, and Fleksy. It happens on all of them. So sure, Android has had this functionality for a while. And some will say that Android having it first is most important. But I think that if Apple does this without having major issues like that, the user experience of using custom keyboards will be better on iOS.