After using Kotlin in Spring projects for a couple of years, I rewrote most of it to Java 17 and stick to Kotlin strictly in Android work. It's not that much different anymore with all the recent additions to Java, and as Java progresses (and the main JVM implementation which is first and foremost being developed for Java) and diverges in other directions in some very important areas (value types, Loom), Kotlin starts to feel more and more like a second-class citizen. I mean, that's what some old-timers were preaching right here on HN from the very start.

Things like Hibernate require some pretty ugly (IMHO) hacks to work properly (you need something like three compiler plugins, and spread keywords around like there's no tomorrow).

The compiler was also very slow compared to Java. The same amount of code written in the same style took 2× times to build. This may have improved.

Probably the only thing I miss are nullability annotations (after getting used to them in C#, not Kotlin).

Edit: after re-reading what I wrote, the tone feels way too critical. Kotlin is a nice language, and certainly easier & more fun to both read and write (although it's trivial to write unreadable mess by going too far with its features). It just doesn't go far enough (for me) to warrant using a separate language with all that entails.

Java has adopted maybe 20% of the things that make Kotlin better, but due to backwards compatibility it'll never catch up.

Personally I think it'd almost never make sense to adopt Java over Kotlin for any greenfield JVM project, unless the devs you hire are truly so low-skill they can't pick it up.