People go into MongoDB expecting the same guarantees for data integrity as PostgreSQL provides. Of course they get disappointed. Mongo has improved with years, I heard, but I’d still not pick it. Plus, PostgreSQL allows you to have columns with unstructured data. Which means you can still reap all of PostgreSQL’s benefits while occasionally using document store where it makes sense.
I worked twice with MongoDB around that time and hated it with passion – it was easy to lose data with it back then. Nowadays PostgreSQL can do everything MongoDB can, and much more.