The qualities of sets that arrays don't have (and vice versa) are irrelevant to the point of neither being implicitly representable in XML.
You're either providing complex objects as properties or you're providing a list of complex objects. Worse, you can have a combination of both. Without a schema it is not possible to infer whether either or both is happening.
You're either providing complex objects as properties or you're providing a list of complex objects. Worse, you can have a combination of both. Without a schema it is not possible to infer whether either or both is happening.