Taken in isolation they're exactly the same kind of interoperability issue; the difference is that Scala has the language facilities (typeclasses and implicit conversions) to solve that kind of interoperability problem and Kotlin does not. In Scala it's straightforward to write methods that work with both Java and Scala collections (either at a syntactic level by using implicit conversions or in a "true" way by using typeclasses); in Kotlin it's simply impossible to write a method that will work with both options and nullables.