I only worked in a small team so far (3 devs) and had a good experience introducing Scala, but I can imagine that with people who have previous Scala experience and "their own style" can be problematic.
Added to this is that "best practices" have migrated pretty significantly over time, and things that many people thought of as very beneficial are now often seen as problematic (xml support, mixins, large for comprehensions, implicit conversions, etc) but not everyone agrees on these.
Compounding all of this is that the Scala "defaults" can lead to some pretty heinous problems and experienced Scala developers often have opinions that seem to directly contradict what Typesafe is selling (for instance, Akka, Play, the collections library, sbt etc. are all subject of ire for lots of people who have used them extensively).
That said, the complaints about Scala's complexity are not overblown in the least. If you ever had the opportunity to move from one Scala team to another, or to integrate 2 Scala teams that had grown independently previously, you see that even saying you are a Scala team does not narrow down significantly what to expect when opening up the code base. There is so much variance in what is and is not idiomatic Scala, that even the Scala standard libraries are a mix of styles and best practices. As for Scala steering you towards the "Right Thing", I'd argue that the Scala community has not settled on what that is (to the point of schism) and my experience indicates that quite the opposite is true.
As far as IDEA achieving parity with Java, the very concept is laughable. Simple things like safe refactorings aren't, and syntax highlighting mistakes and phantom compile errors are common. While the IDE story in Scala has improved dramatically over the years, to say it is Java level is over selling it.