Yeah so you've actually stumbled across the one major functional divergence from the previous implementation:

In the transformation API's encapsulated in `RKMappingOperation` it would recursively process a mapped collection value to enable Array -> Set, then Set -> Mutable Set.

We don't actually have that sort of functionality. The current implementation is strongly bound to the contract that a single value transformer will handle the input value. This means that if you want to go from `NSArray` -> `NSMutableSet` then it has to be handled here.

I toyed with the idea of supporting a 'pipeline mode' or something similar on `RKCompoundValueTransformer` that would enable it to reprocess a transformed value to allow multiple transformations, but held back because of the added cognitive load and API weight.