There's really only one reason to choose mongo over a competent RDBMS, and that's schemalessness at the database level--which most users of mongo then toss away

There's really only one reason to choose mongo over a competent RDBMS, and that's schemalessness at the database level--which most users of mongo then toss away by adding a schema package at the code layer. If your data is relatively freeform with good indices on stable fields, mongo is hugely convenient, and can be easily structured to scale horizontally. For non-join based aggregation (i.e, aggregation within a collection), mongo is very performant, and works fine for a lot of use cases. I'm using it storing large amounts of simple timeseries data and it's great, largely because my use case doesn't require more complex data structures. As soon as you want to deal with more complex structured data, mongo is worse than any RDBMS. The only viable noSQL solution to complex structures is denormalization, which is sometimes a legit choice. But unless you know your primary use is just retrieving a set of documents based on indexed fields (and many are), get an RDBMS.