I have used Kotlin a bit for a hobby project and it felt like they were 95% done with a 1.0 version. I love the promise of a single code base that can run on the JVM and browser, but it is not all there. Until recently, the API was not guaranteed to be stable. Every one in a while, I hit a feature that is JVM only or does work right in JavaScript. The JS compiler will “helpfully” remove uncalled public functions unless you explicitly mark them with JsExport.
Also, from what I can tell, only InteliJ is the only supported IDE (which makes sense, since they are the language developers). There is an official Eclipse Plugin, but the last time I tried it, it did not work and tried to take the entire IDE down with it.
Having said that, it was very close to complete and I have not worked on that project for a few months, so it could all be perfect now.