Well, there are many excuses. Scala is a messy language* and I have Clojure which IMHO is a better alternative for JVM development.
I used both Scala and Clojure and settled with Clojure since it fits functional programming better. Scala is OOP with a bit of FP. Clojure if FP with OOP in case you need it desperately.
* "messy" as in "let's throw every feature we can think of into the language!"
That said, Scala certainly ain't perfect. It's certainly got its warts. And I'm not going to compare it to Clojure, as I haven't played with Clojure enough yet ;)
As the language designer himself has said: Scala is not a better Java, it is not Haskell on the JVM. I would add that it isn't just SML on the JVM either. It is much better, in my opinion, to embrace Scala for what it is, a powerful, expressive, pragmatic language for industrial use.
Your discriminated unions would be "in drag" as case classes, of course. You would want to be aware of the OO parts of the language, but there's little that obligates you to use them.
Personally, if you don't have dependencies on java and if your goal is to do a pure functional application, I would stay away from the JVM as it just wasn't built for it.
My biggest problem with Scala, to date, is all the optional syntax. On one hand, making things optional sounds nice, and it can be handy... but in Scala, you can have a line of code where there are 3 or 4 different ways to write it. Sorry, don't have a good example handy, I'd have to go through my "learning Scala" workspace to dig something up and I don't have that stuff nearby. Anyway, the point is, I sometimes feel like Scala can easily lead to a situation where one is writing "write only" code. I do not relish the idea of taking a large base of Scala code, written by somebody else, and trying to understand it well enough to work on it.
Probably not if you want to keep that Haskell background. I feel Lisp/Haskell have very different philosophies and you might not like Clojure's.
OTOH, stepping out of your comfort zone is always a great way to expand your views (even if you hate Clojure at the end).
> There's almost no excuse not to use Scala if you're deploying on the JVM.
s/deploying on the JVM/a Java developer/
There's also things like protocols, and Typed Clojure.
The typed module implements a static type annotation system and type checker on top of the dynamic language, which is nice, but that still doesn't mean Clojure is statically typed.