Aphyr has thoroughly explained what is wrong with MongoDB[1]. I'm quite sure that things aren't probably as swell with your tech stack you might imagine. A commenter here has in the past demonstrated how to cause someone using MongoDB data problems by sending too much information too quickly for MongoDB to handle in a targeted way although I can't find the link to it now. I have been wanting to try it myself to see if I could do it too. Firebase uses MongoDB[2]:

"Firebase stores all its data in MongoDB which offers the built-in capability for apps to scale automatically and gives each piece of data its own unique URL stored in JSON documents."

I was going to see if I can get achieve data consistency problems there for my own firebase app.

Also posts that malign MongoDB frequently make it to the home page [3][4].

[1] http://aphyr.com/posts/284-call-me-maybe-mongodb

[2] http://www.mongodb.org/about/production-deployments/

[3] http://www.sarahmei.com/blog/2013/11/11/why-you-should-never...

[4] http://www.aaronstannard.com/post/2013/12/19/The-Taxonomy-of...