I wonder if the ease of use of MongoDB might be part of why this "hating on mongo" trend exists -- it's very easy to setup and use without having to think through exactly what the best way to use it is. Which means it's very easy to do something dumb without realizing you're doing something dumb. The problem is, most of these database anti-patterns aren't visible at a small-scale, they only become obvious once the datasets get large and then it's really hard to change things.

Pretty much any database is going to leak abstractions at some point, there's absolutely no way of getting around understanding how your DB works. I think the problem with Mongo is that it's so developer friendly initially, that it makes it easy to avoid a lot of hard decisions that will eventually have to be made, and when those decisions do come to the forefront, it's often at a much less convenient juncture in the application's lifecycle to change things.