It's always important to put things into context. If you're developing a hello world or a few animations for your mostly static web site, the initial "tax" of Scala.js will always be too heavy. It's not so much the power of Scala as a language, but rather its collections library.

But if you're developing a larger front-end application, that tax is dwarfed by the size of your actual application.

Also, Scala.js can be and is used for other JS-based environemnts, which are less size-constrained, such as Node.js, electron, etc.