I use mostly Scala and Rust and both have significant issues still with incorrect syntax highlighting, refactoring being limited and slowness with larger projects.
I guess it's just that I have a hard time reading a statement like "Features are super slow to be delivered" and even trying to understand where that person is coming from. Like, what general, non-language specific features does he think IntelliJ lacks at this point? Is he upset he can't use it to mail a check to his utility company?
Also, Scala sort of recently became completely revamped and I have only had a not too positive experience with the new version - which does make sense that it won’t instantly be as feature-complete.