When you say JSON is more agile is that because you don't need to write migrations? Do you use a framework that supports migrations like active record or south?

We do not use migrations with JSON.

Adding and removing happens on the application layer with using Option[_] monads in the data model. Add a field means add Option[_], removing a field means making e.g. String an Option[String] - or fill with default values on read. If we drop a field over time we ignore it during writes.

   case class Person(name:String)

adding a job field after some time becomes

   case class Person(name:String, job:Option[String])

and application code needs to deal with it.

Works for us, compared to my previous jobs where we used SQL and migrations.

Writing a load of consistency checks in the application layer sounds the opposite of agile to me. Unless you just want sloppy data.