My other site, http://www.BudgetSimple.com on the other hand is using SQL Server (in the process of porting to MySQL). It would not be a great use-case for Mongo, because there are usually as many update, delete, inserts as there are reads, and instant database integrity and a schema are important.
Anyone that claims a tool is perfect for every problem is probably wrong. You need to figure out the best one for your use case, and load test, security test, performance test, etc... until you have a good guess for the right answer.
Did you ever consider a datasource like elasticsearch? If yes what made you choose mongo?
That probably would have worked as well (don't think I considered that specific solution). Mongo came up on top because of it's wide use (among other things), ie it's pretty easy to find support and lots of stories about how to scale it under different scenarios.