One of the advantages with schema-less design is that it works well for prototyping; you can have a collection of documents with each of the documents of variable structure. You can modify the document structure for one, some or all documents within the collection all without requiring a schema for the collection or each and every document.However, this is also a disadvantage during prototyping; there are no constraints to stop documents within the same collection having variable structure
Rules of the game are simple: The sum of all constraints in a system is constant. The more relaxed one of the component is, the more validations other components must perform.
Original title and link: NoSQL Document Databases: Testing Your Document Structure for Inconsistencies ( ©myNoSQL)