I feel that part of Scala's problem in every project I've been involved with is that it has too many features which means if you're not incredibly careful about specifying exactly what features should be used you end up with an unmaintainable mess.
Whenever I've asked people to go into the specific "too many features" in Scala it always turns out they were talking about library features, not language features. The language is powerful enough that people can write very complex libraries in it. If Kotlin ever becomes popular it will have exactly the same problem.
The funny thing is that even as people say that, every new release tends to look a step closer to Scala.