I'll have to disagree with you here: Null safety alone is such a big advantage over Java that it alone makes it a more attractive choice (it's called the billion dollar mistake for a reason). Additionally it allows you to create way better syntax for your APIs, allows you to interact with generic types better, supports extension functions, supports module visibility, has a much better standard library and has better support for immutability and functional style code.

I'm not saying it's perfect – there is certainly a lot of things that could still be improved (like public being the default visibility), but I don't really see any big disadvantages over Java.

Really the only thing that I could think of that Java has over Kotlin is that everyone is using Java. That means that the knowledge base and know-how for Java is higher than that of Kotlin, but Kotlin is by no means a small language. And it even profits from Java's eco system as you can use any Java library in Kotlin, too (and the most popular ones even offer specific Kotlin syntax for their APIs).

What would be your examples for the disadvantages of Kotlin over Java?