The whole point of having a JSON field is so that it has more structured than just text, otherwise you can just use a text field (like SQL Server does). Also they all support various JSON querying and indexing functions that including subfield access with optional computed properties.

Sure MongoDB has some extra ergonomics for dealing with JSON/BSON data, but how much benefit this really adds is still up for debate. As horizontal scalability becomes more natively supported, MongoDB will lose even more of its benefits.