The Objective-C generator uses NSNumber for all the types that the standard codec uses it for; this is the simplest approach, but in many cases it's not the most convenient for clients, or the most idiomatic:

• Non-nullable arguments that aren't in a collection.
• Non-nullable fields.
• Non-nullable return values.

The last is particularly weird; you basically never see a method in ObjC return an NSNumber representing a boolean, because nullable boolean returns are almost always a terrible concept, and most booleans that one works with in ObjC are BOOLs; you generally only box them if you need to put them in a collection.

This would add generator complexity, but having done something along these lines for the C++ generator (where the exact translation of a type depends on the context) it's not terrible.