High coding and migration cost
Most other JSON databases have limited SQL support, or none at all. To make matters worse, their languages are “imperative,” so the developer must specifically tell the database “how” to gather the data. This makes the migration of applications very expensive because you must rewrite the data access layer and retrain developers. It makes maintenance worse, too.
SQL and SQL++, on the other hand, is a “declarative” language. Simply ask SQL++ what to gather, and the database does all the work.