MongoDB and its hype burned many, many people back in the day. It's hard to overstate how much it used to suck ass. At one point MongoDB client libraries would report that writes succeeded even if your box had no internet connection, that's how bad it was.

The Mongo fans claim "no really, it's good now", but I was one of the people burned so badly that I'll never touch it with a ten-foot pole again for as long as I live. I assume this toxic reputation is a big part of the problem.