The issue isn't that mongodb isn't suitable for some problems. I've used it before and it has not caused issues. It's that the problems it is suitable for are a subset of those that postgres is suitable for.

I can just as easily rapidly prototype in postgres. If I want to store schemaless JSON in postgres I can easily do that.

>In the end, I think it is good to have a selection of tools that are good at doing different things.

In the end I think every tool needs a niche - something it is good at that its competing tools are not good at.

Mongo doesnt have that.