No idea how stable it is but IMHO html builders are nowhere near the jsx in convenience and ease of use. Typesafe - yes, comfortable - no.
Using hyperapp.h/ReactDom.createElement like api in kotlin is less verbose than using builders.
I would suggest using full kotlin for business logic, dto’s, server communications and lot of dynamic for actual view components with some in project api wraps for templates.
If you have no experience with kotlin.js expect to have multiple minor/major bumps while getting used to toolchain. Like not so fast build times, provided client apis and types being of latest and greatest version but not actually supported for like 50% of browsers, problems while debugging, like no easy way to inspect some collections etc
In general you have to understand what actually happening both on js and kotlin sides otherwise it will be an adventure.
I have about 2 years worth of kotlin.js experience and overall would recommend it. But having deep understanding of js and browsers is absolutely required.