We use it for backend. F500 company.

There is almost no locking with Kotlin. You can stop writting Kotlin code any time and start writting Java code.

However I think it's not possible to call coroutine code from Java.

Exactly why we use it. Our java devs have been able to pickup and start writing kotlin. None of them have regretted it that I've seen.

It's extremely close to Java in terms of most concepts. That makes it an easy language to switch into and out of. It's java with a nicer syntax and more sugar.