Umm... why would you not choose a relational engine to start with? This isn't said with the benefit of hindsight. I worked with Lotus 123 back in the early 90s. I get that there's value in document databases, but even then, there were limitations, and the need for the ability to have structured/related data in an engine that easily allowed for ad-hoc queries out of the box was apparent.
I watched the entire nosql/mongo movement arise and evolve over time, and it rarely made sense to me. Even when mongo made sense for a problem... it only makes sense for that problem, and not as the primary basis for your entire application. Relational/SQL, with nosql/mongo/etc as secondary/ancillary data store has always made the most sense (imo).
JSON columns in major databases now tend to provide a pragmatic balance of "good balance for many use cases". Justifying something other than that as a starting point is possible, but I've rarely seen cases where it makes sense to avoid a decent SQL engine as the core data store for your business apps.