That's not always true, of course. NoSQL databases can usually handle 1st level relationships just fine. The problem arises when the related item needs to be it

That's not always true, of course. NoSQL databases can usually handle 1st level relationships just fine. The problem arises when the related item needs to be it's own entity. For example, if you have a table of users who have permissions represented by text, that's really easy in a NoSQL database, as most of them provide some kind of list datatype (list<X> in cassandra, arrays in mongo, list in redis). The problem arises when consumers of the database need to be able to add or remove from the set of possible permissions. If it's limited forever to 'read'/'write'/'execute', or is unlimited, you're golden! But if it's some sort of system where an admin needs to be able to dynamically create customized permissions, an RDBMS will be the clear cut winner.