> Scala has way more language features than Java

I wouldn't say that. I think the main difference is that Scala spent its language design budget on more useful things compared to Java. Have a look at C# if you want to see a language with tons of features.

> What I mean is, how much do you know about what a particular piece of code does when looking at it in isolation? How much additional context do you need to figure out what it does? How many possibilities are there?

From experience, that amount of code is considerably smaller than Java because I can be pretty sure that some piece of Scala code usually won't mutate stuff all over the place.