As I described on another reply, SpreadOn not only supports a tree structured format, but also has tags for referring to entire sheets by name, for declaring grids of identically-typed data, for referring to named regions and extracting subsets of them, and compactly defining tables of repeating structures, which you can mix on a case-by as they make sense. (You can't embed a grid in another grid since it requires more than one cell, but you can make a grid of sheet references, since the sheet name only takes one cell and can be put into a grid.)
There is no "one best way" to represent JSON data in spreadsheets, because JSON data comes in all sizes and shapes, so SpreadON tries to support many different useful formats that you can link together, with a simple straightforward syntax that can be easily extended without breaking existing documents.