Scala has much worse tooling (SBT is an oxymoron), and it's much harder to read than Java.
Conceding that it's more fun to write, I think 'harder to read' is more important. Scaling refers to team size and amount of functionality in addition to requests per second.
The "much harder to read" varies greatly between codebases. Most "sane" codebases are 90% (random percentage) as easy to read as Java. Just collection initialization, transformations, simple pattern-matching, method invocations...
Some times there is an ugly nested flatMaps/maps/filters. But in my experience, in general, they're more the exception than the rule.
Scala compilation is indeed slow (although I think C++ is worse). But I always have a "sbt ~compile" running, compiling every file as they change. So the compilation time is pretty much irrelevant to me. While using eclipse, I guess, you'd get the same speed too.
