Assuming it's for the JVM, you should definitely choose Kotlin for your next project. Most of the flaws I find are for advanced usage (e.g. interop, coroutines, etc) or tooling (e.g. gradle issues, dokka) or just general advanced language use. I don't keep a running list of issues (I made one back in the day [0] but not all of it is still applicable), but nothing would keep me away from the language. It made a lot of right decisions.

0 - https://gist.github.com/cretz/2a49514b18914ef09b7c518db6db11...

Jetbrains has a very good 4 weeks free course on Coursera https://www.coursera.org/learn/kotlin-for-java-developers. We’ve beginning to introduce Kotlin in our code based, and also begin to use Kotlin Dsl on our new Gradle projects; for the moment, the experience is excellent. Interop with Java is really good and the support from IntelliJ is also fine.