Earlier quoted context omitted.
Why wouldn't you always use something that is tried-and-true? MongoDB is relatively new compared to something like postgresql or even mysql.
If you actually need certain features like a document store, then Postgres is less mature than MongoDB in that regard.
Unstructured documents are very rare. Querying arbitrarily against them even more so. For the fields that do need querying in a document, put them apart and add a few indexes and you're good. That's been my experience and I've been around a while.