Maybe it's not what you looking for, but Immutables support binary [1] and JSON serialization [2] (Jackson or Gson) very well including field naming customization.
>> what immutable has that checker framework doesn't.
Functionality overlap is not particularly clear with Checker framework, in fact Immutable users often use Checker framework also. There are other tools like AutoValue, FreeBuilder, Lombok where I can see more similarities, but with important differences.
>> Looks like immutables transform the code in even more spaghettis.
I would not agree, but you should look at real world codebases like [1] or [2] which use immutables.
>> Just looking at the "get started", it looks like the author voluntarily ignores all the java conventions around variables and getters/setters.
Immutables support getters convention and you can configure any other convention you have, see [3].
>> I might be wrong on this, but I suspect this framework heavily relies on introspection, which in my experience has always led to very hard to debug and explore code.
Immutables uses standard APIs of annotation processing. You can take a look at example generated code [4].
[1] http://immutables.github.io/CompareImmutables.pdf
EDIT