That depends. If the rest of your application is in Kotlin, you're reducing cognitive overhead. Same language, same compile, same distribution. You can share view models between frontend and backend. It's much closer to what GWT promised to be than GWT ever was.
> I don't buy into sharing source files instead of clean separation via Web APIs, the additional tooling complexity isn't worth the pain.
Fair enough, that's a different argument from the one you posed upthread though. The ecosystem and tooling are very much there.
Happy to disagree on architectural merit of the approach. Personally I'm done with javascript/typescript for any future products/teams I build, and am glad to have a viable solution.