IntelliJ is OK for most Scala code, but it regularly makes a mess of anything complicated. So much so that, in our office, the first answer to "why doesn't this code compile" is "Does it really not compile, or is IntelliJ being dumb again?" It's usually the latter.

I think it depends on what style of Scala you use. Anything involving slick or spray tends to confuse IntelliJ. Its better in 14, but it's still not good.

The refactoring is bad enough that I hardly ever bother using it. It gets simple things mostly right, but doing that by hand is just as easy. It can't do the more useful things like method signature changes.

The tooling support for Scala is improving, but its at about 10 years behind Java tooling.