There's probably a good use-case for MongoDB, but what I saw in practice was that it was an excuse for poor developers to avoid learning some dialect of SQL.
It's the same story with ORMs. You will eventually need to learn to write SQL. For some reason developers try to avoid this almost as much as possible. Maybe more than other technologies.
At some point, you are going to want to query data. And eventually you will reinvent what RDBMSs do out of the box, because they're purpose built for this work.
Further, if you need to just dump JSON, an RDBMS is still a better choice for all the reasons above.
Find me all users with X... (SELECT * users WHERE X,...).
In practice, I've found that you don't really query documents. Can you? Sure, I suppose, but I found myself reinventing what those databases do for me.
Sorry, I know that's not a formal explanation. But it is a practical day-to-day one. It's easier for me, and I think others too albeit with practice, to simply write a line of SQL to get what I want.