I have 3 large applications built in Kotlin (only the backends though), it's been a pleasure building them in Kotlin and they've been running without any issues for over a year. Also don't get random null pointer exception failures like with a typical Java application, my experience so far has been that it was rock solid after the first deploy whereas with Java applications, you often do a 3rd and 4th deploy to fix null pointer exceptions you haven't checked for under certain conditions.

I'm planning to use KotlinJS in the future for frontend dev too, just waiting for it to become a little more polished. It's already stable enough though for starting a new project.

I don't have anything with Kotlin in production. But locally, I'm working on a kotlin-common project (JVM+JS) and that's going well.

There's not much browser integration, it's mostly isolated logic some a big of platform specific stuff. For that limited use case, I've not had any problems. It's stable and expressive and indeed, much better than Java.

I do get the impression JVM-Kotlin is much more popular and easier to find information for.

TLDR; The language is great, the JS backend works, but I don't know much about dom manipulation or integration with js libraries (other than qunit).

Check below guide on Kotlin for beginners. Hope your doubts will be cleared.

A Beginners Guide to Kotlin

Hope this helps !