What are the disadvantages to declaring Scala case classes?

First the good bits:

Everything immutable by default

Yes, and can even be overridden (using var) if you need it

Getters automatically defined

Possible in any class by prefixing params with val

Decent toString() implementation

Yes, very useful, but doable by hand on any class if necessary

Compliant equals() and hashCode()

Combined with easy pattern-matching, this is the main reason that people use case classes

Companion object with unapply() method for matching

Also possible to do by hand on any class by using extractors

This list should also include the uber-powerful copy method, one of the best things to come to Scala 2.8

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Then the bad, there are only a handful of real restrictions with case classes:

You can't define apply in the companion object using the same signature as the compiler-generated method

In practice though, this is rarely a problem. Changing behaviour of the generated apply method is guaranteed to surprise users and should be strongly discouraged, the only justification for doing so is to validate input parameters - a task best done in the main constructor body (which also makes the validation available when using copy)

You can't subclass

True, though it's still possible for a case class to itself be a descendant. One common pattern is to build up a class hierarchy of traits, using case classes as the leaf nodes of the tree.

It's also worth noting the sealed modifier. Any subclass of a trait with this modifier must be declared in the same file. When pattern-matching against instances of the trait, the compiler can then warn you if you haven't checked for all possible concrete subclasses. When combined with case classes this can offer you a very high level level of confidence in your code if it compiles without warning.

As a subclass of Product, case classes can't have more than 22 parameters

No real workaround, except to stop abusing classes with this many params :)

Also...

One other restriction sometimes noted is that Scala doesn't (currently) support lazy params (like lazy vals, but as parameters). The workaround to this is to use a by-name param and assign it to a lazy val in the constructor. Unfortunately, by-name params don't mix with pattern matching, which prevents the technique being used with case classes as it breaks the compiler-generated extractor.

This is relevant if you want to implement highly-functional lazy data structures, and will hopefully be resolved with the addition of lazy params to a future release of Scala.

I think the TDD principle apply here: do not over-design. When you declare something to be a case class, you are declaring a lot of functionality. That will decrease the flexibility you have in changing the class in the future.

For example, a case class has an equals method over the constructor parameters. You may not care about that when you first write your class, but, latter, may decide you want equality to ignore some of these parameters, or do something a bit different. However, client code may be written in the mean time that depends on case class equality.

One big disadvantage: a case classes can't extend a case class. That's the restriction.

Other advantages you missed, listed for completeness: compliant serialization/deserialization, no need to use "new" keyword to create.

I prefer non-case classes for objects with mutable state, private state, or no state (e.g. most singleton components). Case classes for pretty much everything else.