There was a spate of high profile "why we moved from MongoDB to $other_database" articles a year or so ago of startups getting burned when they realised that the all-signing all-dancing cure-all database they (thought they) had been sold, in fact wasn't.
The issues encountered ranged from MongoDB failing to scale out as easily as promised, to significant loss of data. There was also some backlash because MongoDB didn't (and perhaps still doesn't) persist data to disk when it acknowledged it as received[0].
A general perception grew, rightly or not, than MongoDB was being marketed by 10Gen[1] over and above its capabilities. Then the actual message of "MongoDB is not a drop-in replacement for traditional RDBMSs like PostgreSQL and MySQL, and is not the ideal solution to every problem" has been filtered down to "MongoDB is a terrible product with no use cases whatsoever." Such is the effect of the HN echo chamber.
Much like PHP, MongoDB is now simply a product that you cannot say anything good about on HN, even if you are in fact finding it an effective tool for your use case in spite of its shortcomings.
[0] Cynically, because they were trying to win benchmarks. Less cynically, because the use case was semi-ephemeral data where possibly losing some data is acceptable.
Every DBMS comes with its own set of special problems. Most people only find out about the truly tricky ones once they have deployed a database in production (i.e. when it is too late to decide your needs are actually not served all that well by what you chose).
Once a DBMS like MongoDB gets popular, then people uncover some of its failings and write about them...which is great because it means you don't have to discover the same things the hard way. But it's a drag too because there's no database system that everyone agrees is adequate (much less ideal) for every application.
Other NoSQL solutions were not marketed as a replacement for relational databases.
MongoDB might be better now,but you cant take managers for idiots an expect them to buy into your product again. Right now MongoDB only lives because of javascript and nodejs ,since nodejs sucks big time with anything that is not MongoDB,library and driver wise.
The issues encountered ranged from MongoDB failing to scale out as easily as promised, to significant loss of data. There was also some backlash because MongoDB didn't (and perhaps still doesn't) persist data to disk when it acknowledged it as received[0].
A general perception grew, rightly or not, than MongoDB was being marketed by 10Gen[1] over and above its capabilities. Then the actual message of "MongoDB is not a drop-in replacement for traditional RDBMSs like PostgreSQL and MySQL, and is not the ideal solution to every problem" has been filtered down to "MongoDB is a terrible product with no use cases whatsoever." Such is the effect of the HN echo chamber.
Much like PHP, MongoDB is now simply a product that you cannot say anything good about on HN, even if you are in fact finding it an effective tool for your use case in spite of its shortcomings.
[0] Cynically, because they were trying to win benchmarks. Less cynically, because the use case was semi-ephemeral data where possibly losing some data is acceptable.
[1] Now also called MongoDB