The problem with MongoDB is their shadiness. The shipped with unacknowledged writes up until not too long ago. In other words you would write to it and there wouldn't be an ok or fail response, you'd just sort of hoped it would go in.

They fixed that problem but it was too late. In my eyes they proved they are not to be trusted with data.

Had they called themselves MangoCache or MongoProbabilisticStorage, fine, can silently drop writes, I don't care it is not database. But telling people they are a "database" and then tweaking their default to look good in stupid little benchmarks, and telling people they are webscale, sealed the deal for me. Never looking at that product again.