People seem to forget the MS in RDMS.

It's true that anytime you write a function that maps some set to the set of logical values (true, false, null, maybe, ...) you're dealing with relational data. So while there is no clear cut border, at some point it becomes obvious that you would benefit from a Relational Data Management System. The danger is that your particular data set might out grow a particular management system.

When you switch to a "NoSql" solution all you're doing is deciding that you need a more purpose built managment system for your relational data.