Sure, MongoDB has its shortcomings, but so does <Someone's favorite database>. It's much more productive to understand the pros and cons of using it and use it (or not user it) appropriately.
I've been learning Postgres lately and have been liking it so far. What are its shortcomings? Haven't run into something bad about it yet.
Though it has definitely been improving over the years.
There are third-party tools which add multi-master replication, but all seem to suffer from some combination of complexity, flakiness, project deadness, or project immaturity. Postgres-XC seemed the best last time i looked, but it does seem quite complex.
Work is currently underway to bring multi-master replication into core PostgreSQL, under the name "bi-directional replication":
https://wiki.postgresql.org/wiki/BDR_Project
I have no idea if or how this interacts with Postgres-XC. I would imagine the end-game will be that by ~2016, there will be both tightly-coupled (synchronous, immediately consistent, like Postgres-XC) and loosely-coupled (asynchronous, eventually consistent, like BDR) multi-master replication in core PostgreSQL.
That's a long time to wait given that there are noSQL and newSQL databases which have that right now, but when it arrives, it will bring PostgreSQL's huge baggage train of other good stuff with it.