It's less costly to create a relation table when you realize there may be multiple instances of a piece of data associated with a record than it is to just stick those pieces in an array.
Because when you don't use the relational data, you get the extra work of modifying all of your existing NoSQL records to use the array structure. And as a bonus, you make it easy to do queries in both directions with the relational data.
NoSQL offers virtually no efficiency benefit unless you're actually consuming unstructured and variable data.
You'll easily make up this lost time just in not having to immediately clean up crappy data that you've written to the database while you're developing the feature. That's been my experience anyway.