I tried writing Kotlin in Vim, I really did. Java was somewhat manageable, but Kotlin was absolutely impossible. The biggest thing those bloated Java-based IDEs offer is deep introspection into the language. You use a reference, the IDE can auto-import the package for you. When learning a new language, knowing what package to import is invaluable. Even the Kotlin in Action book from Manning Publications flat out says, "the IDE will automatically import that for you". It was quite a turn-off but.... it's a nice language, so when I write it, I have to use IntelliJ.