So does PostgreSQL, but I have no idea about MySQL. You can run xpath queries against XML documents, and you can also index specific xpath queries (there is no general indexing infrastructure for XML in PostgreSQL). For JSON no path lookup functions are included in the core yet, but I assume there are extensions which add them.
If you need to store multiple forms types, it gets hairy very fast.
MongoDB allows to query inside the data (which is not a blob in that case).