MongoDB is fine at the conceptual level. The problem is that the architecture and implementation are consistently poor in myriad ways. It is a case study of what can happen when well-meaning individuals with little experience in database architecture and implementation attempt to build a scalable database engine.

That said, a competently engineered RDBMS can do everything NoSQL databases can, particularly limited databases like MongoDB. The caveat is that you have to learn how to use those databases; they are very feature rich and powerful but that flexibility makes them more complicated. PostgreSQL is a very good choice from the open source world and is just as fast as NoSQL du jour in the hands of someone that knows it.

I currently design extreme-scale real-time analytical database engines, so I have no vested interest in any particular solution (we are not really competing with the current market). If I was going to build a large-scale web app today and needed a backing database, I would go with PostgreSQL -- it is very capable and well-engineered.