Also, Scala is usually a bit slower than Java (let's say factor 1.5x) which will be a problem if you need raw performance/latency, e.g. if you want to write a high frequency trading app. You will not be happy doing that with Scala.
As to why I believe Scala is more productive - just look for example at Typescript and how much it improved developer experience by supporting union-types. Scala allows to do the same. Or think of Rust and its traits that make it really easy for library interoperability - Scala has the same (in an even better form). Last but not least, Scala is currently the language with one of the best effect systems in my opinion (https://zio.dev/). Kotlin for example has copied the approach with https://arrow-kt.io/ which I think is great actually. But when comparing Scala and Kotlin here, Scala wins by a large margin, it is a completely different world. It's like building a highly concurrent system in Erlang vs C.
Of course, if you don't want to learn things like union types, traits/typeclasses and effects (similar to async/await but more powerful) you will be annoyed by Scala. But once you learned them, you can never go back.