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Could you be more specific? Is the compiler promising more than it can deliver, or are you saying the compiler could check something but simply doesn't?

You can have 100% type safety in a straight jacket, but scala pushes what it tries to type check to an extreme limit; "safe" but then also not as conservative as Haskell or ML. I see scala as the c++ of statically typed languages, filling that niche where you need more from your managed language and are willing to take a leap, but this is controversial and a personal opinion.

Do you have an example of code that will throw an unexpected ClassCastException?

I never, ever had a ClassCastException in Scala that I wasn't warned about by the compiler. Maybe in theory you are right. But what counts is practice.