What's funny is that on all but one of your "Mongo is good at" points, Solr beats the snot out of it. The only debatable one is ease of starting up, which is a silly criterion when talking about a project that has any sort of lifespan. Even then, Solr startup is really quite easy.
Obviously Solr uses special-purpose indexing that one might argue is a bad fit in some cases but if you're using Mongo anyway...
Postgres is good but its horizontal scaling "story" is not great, obviously. But finding the edges of Postgres's goodness will take much longer than to bump into Mongo's limitations.
What's funny is that on all but one of your "Mongo is good at" points, Solr beats the snot out of it. The only debatable one is ease of starting up, which is a silly criterion when talking about a project that has any sort of lifespan. Even then, Solr startup is really quite easy.
Obviously Solr uses special-purpose indexing that one might argue is a bad fit in some cases but if you're using Mongo anyway...
Postgres is good but its horizontal scaling "story" is not great, obviously. But finding the edges of Postgres's goodness will take much longer than to bump into Mongo's limitations.