What's the core tradeoff of storing data in a JSON column instead of proper relational columns?

A JSON column gains schema flexibility — new fields can be added without a migration, useful for genuinely variable or evolving data. It loses the relational model's structural guarantees: no database-level enforcement of the JSON's internal shape, and querying/filtering/joining on a nested JSON field is generally more awkward and less broadly indexable than an equivalent relational column.