The only practical application of MongoDB I can justify to this day, is if you have a form builder, that allows users to build completely custom forms. Forms.io

The only practical application of MongoDB I can justify to this day, is if you have a form builder, that allows users to build completely custom forms. Forms.io gives you the form schema in such a scenario as JSON out of the box. That gets matched with answers as JSON. Saving this to MongoDB directly, rather than SQL seems to simplify things. With anything else, at the end of the day, you are enforcing FK constraints anyway, so might as well use SQL. I never had issues with MongoDB performance. One caveat to this is that I am yet to see a project that needs database sharding in real life, and I have worked on projects with millions of entries in a table and hundreds of writes a minute.