Just that Java libraries aren’t going to be taking advantage of Kotlin-specific language features, which in turn makes it harder to use those features in the pa

Just that Java libraries aren’t going to be taking advantage of Kotlin-specific language features, which in turn makes it harder to use those features in the parts of your application code that interact with those libraries unless you write wrappers that make these libraries fit in better with your Kotlin codebase. There’s a similar issues with using Objective-C and C libraries in Swift, which makes Swift libraries more appealing where available despite Obj-C/C interop being good.