I see many features that would traditionally be better modeled with a relational db. Could you share some of your successes or difficulty with using MongoDB?

I see many features that would traditionally be better modeled with a relational db. Could you share some of your successes or difficulty with using MongoDB? Its the number one thing that turns me off from Meteor every time. Manually managing your joins seems like a real pain, especially considering how easy other frameworks like rails make it out of the box.