[Tainnor]

I'm also a bit skeptical about Kotlin multiplatform, but I haven't seen it become an issue so far. Spring e.g. fully supports Kotlin (and so do tools like Gradle) and I've never had any issue with things not working.

Calling Java code from Kotlin may not always be 100% idiomatic but it's still by far the best interop between two different languages that I've ever seen (compare that e.g. to Scala). The interop is more than good enough to be viable for a migration scenario where old stuff is written in Java and new things are written in Kotlin - I definitely wouldn't recommend keeping writing both new Java and new Kotlin code, though.

Build times can be an issue (though hopefully improved with the new compiler), but incremental compilation helps (something that maven unfortunately sucks at, so it's better to use gradle). And in any case, the compiler does more (useful) work in Kotlin, so I think it's ok that it takes a bit longer.