I tried to use Scala for Android in the past and I gave up after attempting to set it up for a few hours. Admittedly it was a while back, perhaps things have improved.
Your claim that Scala on Android works fine got me interested again, so I googled around, found that blog post from May this year (that's more recent than my own experiments), and apparently there are still hurdles: http://blog.scalac.io/2016/05/19/reflections-on-starting-and...
With this type of issues the thing that I'm wary about is not getting a "hello world" app to compile once, but that I can get a project up and running, then need to add something else to the equation (say, a DI, or some code processor), and things fall apart again in an X-Files fashion.
I've heard that Scala has long build times, too, but maybe it's a thing of the past as well - no first-hand experience here as I never got to that stage.
Just use sbt-android and skip all the other nonsense.
Compilation is impressively fast, as the plugin uses the information from incremental compilation to skip ProGuarding completely for things that haven't changed.
Kotlin is missing all of the most powerful Scala features. No type classes, monadic comprehension, higher kinded types, adhoc polymoprhism. :(