I hear this a lot about Scala, and I am a Scala dev so maybe I'm biased, but I don't get the difficulty angle particularly. I'm in no way exceptional as a programmer or IQ-wise but I picked up Scala easily as part of my first job. The difficulty thing doesn't compute to me but it could just be that I'm well-suited to the language. It works beautifully as a better Java if that's how you wish to use it. Granted, it might be hard if you start with a "pure FP" codebase but most devs don't struggle picking things up on most code bases even as juniors.
The point about tooling and developer experience I'll concede. It has been at times patchy in Scala in the past but it's pretty good these days.
As I say, I have no hate for Kotlin, and I'll possibly use it one day being a JVM developer familiar with Java as well, I just find it unfortunate that a language which is so similar to the languages it's influenced by was invented.