Not true. We're processing 12TB of data each month with thousands of writes per second and sub-millisecond response times. You simply have to understand how to use MongoDB correctly which isn't difficult with the out of the box settings and by reading some of the documentation. Figuring out your working set is the most difficult part but that doesn't take long to calculate based on understanding the queries you're doing and creating the appropriate indexes.

Since you seem to be fairly negative about MongoDB but light on details, perhaps you should write up your experiences so others can learn from what you did wrong.