Let's take a look at it from a different angle.
Say, I have a group of developers who have been trained to use Java (both best-practices, idioms, tools, etc).
Let's say that JEE 6 shows up (no, I don't remember JEE 2.1, what is that? is that thing even exist?).
JEE 5/6 introduces a few things on top of my head (please pardon me of the syntaxes):
1) Dependency Injection via annotation (CDI in JEE6, EntityManager in JEE5)
Introduction: annotate the instance property you'd like to inject and GlassFish will magically give the class to you.
Example:
@Inject
AccountService accountService;
2) JAX-RS
Introduction: A class, that works like servlet, but without doGet() or doPost(), instead you have methods that maps to the request-path based on the annotation.
Example:
@Path("/account/{id}")
public Account get(@PathParam("id") final long id)
3) Transaction (in EJB3 which appears in JEE5)
Introduction: Sprinkle annotation in your class or method, you get transaction magically.
Example:
@TransactionAttribute(REQUIRED)
public class AccountRepository
or
@TransactionAttribute(REQUIRED)
public Account updateAccount(final Account toUpdate) {...}
So I'm not sure how complex all of these to grok, granted if you need to do something out of the ballpark, you may have to learn a bit more of each individual component but to get started and be productive seems quite easy for me.
Now let's compare that with picking up Scala from zero (given that you know Java already) and get comfortable and productive with Scala, its tools, and its libraries to build web-application with the similar setting of that in JEE (web-service, REST, persistence/db/orm/transaction layer, unit-test, integration-test, etc).
No cheating though: no calling/utilizing JEE. Has to use all-things Scala libraries.