Then
W3C uses it in banana-rf: github.com/w3c/banana-rdf
@jducoeur builds Querki : github.com/jducoeur/Querki
TypeSafe shares a very substantial template: typesafe.com/activator/template/play-scalajs-showcase
All of the above demonstrate rather advanced features & techniques.
The careful reader can learn a lot and determine for himself it ScalaJS is ready for prime time.
Lastly, this https://github.com/SemanticBeeng/play-scalajs-showcase is a derivative of the original showing how to apply event sourcing and functional domain models to move from a CRUD oriented API to a functional API.
This is an experiment and work in progress.