I am really a beginner of both Scala and functional programming. I took the first Coursera course on Scala, and I got really frightened when I noticed that you can express many things in either the functional or in the object-oriented paradigm. One example is pattern matching vs. polymorphism. When you mix up those ideas throughout your code base, it might become really hard to understand.

This is just a thought that I had, and I wonder if that's actually a relevant issue in practice.