This is so very true. I find Scala tends toward the classic "read-only" joke. You can write very powerful code, but it's often hard to debug and it tends to be quite difficult to read down the line. We have a Scala application that is being slowly replaced, but which I maintain in the meantime, and it is by far my least favorite job duty, despite that I theoretically like Scala more than, say, Java.

Probably my biggest disappointment with Scala is really the type system. It's incredibly powerful, but it's unwieldy to use, and it's still full of footguns. "object" is great and useful until someone accidentally turns it into a giant race condition (true story). Traits can easily turn into a maintenance nightmare (ever seen an 8-way multiple inheritance scheme that fans out into 23 separate ancestor classes? I have!) Exceptions end up in a strange territory. There's ways to handle all of this (much of it involves "don't do that!"), but the point is that for all the type system's powers, it's not actually buying you that much except in comparison to Java 6/7. If I want powerful type magic, I can use Rust or Haskell or similar and get stronger guarantees from my compiler. If you're writing "better Java", which is traditionally one of the more common Scala uses, it's harder to justify over Java 8 or Kotlin these days.

All of which is to say, Scala is a really cool language, but I wouldn't write new projects in it if I didn't have to.

Bonus round: We had a Scala iOS app via RoboVM before RoboVM support was yanked. It still gives me nightmares.