It seems like they tried to put all features they could in there without thinking about the orthogonality of the result.
Languages that need all that restraint to be used right maybe are ok for some very clever people, problem is all the rest of us that can't juggle all that in out heads.
Scala tried to be (1) flexible and (2) very interoperable with Java.
This lead to (1) a lot of confusing intersections of paradigms and (2) a lot of compromise even within those ideals.
That said, I don't see any better statically typed functional language with even half the available libraries of Scala (because of Java), so I stick to it as my language of choice. Haskell is very close though, with the added benefit (IMO) of native compilation.
I really think simplicity is underappreciated in language design. Packing a bunch of features into a language can make it powerful, but it also makes it difficult to reason about. This in turn makes it harder to maintain code and collaborate with others.