I've promised myself to never touch MongoDB again.

Worked for a valley startup back in 2013 that picked Mongo as primary data store because the CEO liked the simplicity and was too busy with the future to learn anything more complicated.

I only implemented a couple of features before I got out of that mess. But from my experience, compared to dozens of SQL and NoSQL databases I've worked with; it was definitely the worst option. We spent way too much time dealing with Mongo-specific issues and limitations.