The major issue with the document model is storage. How big can a document grow? How do you persist it, and make small changes inside it? How do you handle "hot" documents that are very popular? Moving data between documents or having part of a document reference part of another document are complex. Complexity comes from building derived data that looks at slices of a document.
With a block-oriented model, your records are much more manageably sized. It's easier to reference or move data between documents, but in turn, you need to do these kinds of recursive shenanigans because your individual records are smaller in scope. Complexity comes from derived data that composes blocks together into a document.
