You are right - the schema for SQL databases is pretty simple. I wouldn't say document DBs are preferred over RDMBSs, it's just that Para is a schemaless backend server so the data fits better inside a document DB. In other words Para expects data in any kind of structure.
One potentially important consideration is that with MySQL, there is a limit of 64k for any given column. This in turn limits the total size of the post and associated metadata. We're moving to Mongo (AWS DocDB, actually) to address this, as Mongo supports document size up to 16MB.
Gotcha! Thanks for the reply!