Mostly good. We use the Fritz2 framework. It's a reactive UI framework that has a nice component library and supports styled components using a Kotlin DSL for CSS. It heavily leans on things like Flows (from Kotlin's co-routines library). We have a multiplatform client library that uses kotlinx-serialization, ktor, kotlinx-date, and a few other things.

Interoperability with javascript is possible but a bit cumbersome due to the need to map types. A typescript mapping conversion exist but it does not really work that well. We use e.g. the maplibre framework to display maps and it has issues converting the type mappings for those. We ended up doing this manually for the bits of API we needed.

Main downsides are the relatively immature tooling (though that has rapidly improved over the last year) and slowish build speeds. The Kotlin compiler has some performance issues.