Pardon my ignorance but I was hung up on this line.

Out-of-sync document stores could lead to subtle bugs, such as a document being present in one store but not another.

But then the article suggests to upload synchronously in S3/DDB and then sync asynchronously to actual document stores. How does this solve out of sync issue? It doesn't. It can't be solved is what I'm thinking.

Data, numbers

How much data are we talking about?