Kotlin is a really good language. I really want it to be my primary language for android development. The only problem I have now is build times. It's about twice as slow to build a Kotlin app than Java app.
Big programming languages are built from big JARs (compiler, runtime, stdlib). Loading those each time in memory is a long process. Using nailgun saves ~10 seconds on my laptop.
Another problem is compiler performance (both Kotlin and Scala compilers compile less statements per second than Javac), but it's not that dramatic.
Finally, dexing languages with runtime (kotlin - ~200KB, scala - more than 1MB) takes time.