I've been using Kotlin in the backend since 1.0 (4 years now??) and it's great, but it does have a few small annoyances. We still have most of our codebase in Java and routinely, we create new Java code instead of Kotlin for everything, interestingly enough (mostly because when a module is already written in Java, it's mostly a source of confusion to add a few Kotlin files - so we tend to only use Kotlin when creating new modules that are fully Kotlin). Also, some of our developers just don't seem to have enough abstraction power for the higher level of code you can write in Kotlin (specially when you start using things like Kotlin builders to make code "prettier", or just abust a little bit the "everything is an expression" feature to completely avoid separete statements - e.g. functions without braces).

People moving from Kotlin back to Java is not unheard of: https://blog.allegro.tech/2018/05/From-Java-to-Kotlin-and-Ba...