Kotlin is interesting as a middle ground, but I still find it much less productive than Go for most tasks, and unsuitable for tasks where you'd reach for Rust.

In practice, Kotlin is extremely complicated, and you end up spending time being clever. There are 1000 ways to do things. Operator overloading. Proxies. Properties. Companion objects. Exceptions AND result types...

The build system (assuming you use Gradle) is tantamount to torture for anyone used to "go build".

The coroutines APIs feel simultaneously more complicated and yet more restrictive than Goroutines. More structured but less flexible and more effort to use.

Access control feels awkward. There's no way to make a type package-private -- it's file-private or available to the whole module. This leads to either a larger API surface than desired, or the inability to break up complexity into multiple files.

Kotlin/Native and Kotlin/JVM really are two different beasts too.

Kotlin/JVM is mature, but then you are running on the JVM, so that cuts out a whole class of use cases you might bust out Rust for.

There is a very weak ecosystem for Kotlin/Native, and it's poorly documented. There are some scary bugs in the bug tracker.

You can't publish source-only libraries for Kotlin/Native either, so you need a complex CI setup to build binaries for every OS and arch under the sun. Or just don't publish libraries at all, which probably feeds in to the weak ecosystem...

Don't forget that it's made by someone trying to sell you an IDE!