You can program in pure mode in scala too(and scalaz framework helps in it). FFI is great but i don't miss it. For better performance there is the java collections and libraries.
Scalaz helps with the idioms but without the purity garuantees you miss out on all the optimizations and verifiability.
The java collections & libraries don't help me when I need to manage my own memory. I've spent entirely too much time in the unsafe package because the JVM doesn't allow me to do cheap ffi unlike Haskell or even Microsoft's CLI.