Whether it is an object, array, something else, or a combination thereof is a design decision. It is not so much about the design of the structure, which should be determined by execution performance considerations, but how information is added, removed and retrieved. Gathering one or more records from a JSON object, or array index, by value of some child property somewhere in a descendant structure of the instance index always feels like a one-off based upon the shape of the data. That could just be a query which is more elegant to read and yet still achieves superior execution performance compared to a bunch of nested loops or string of function array methods.

The more structures you have in a given application and the larger those structures become in their schemas the more valuable a uniform storage and retrieval solution becomes.