If you actually tried using it as one you would realise how ignorant your comment is. Both the Solr and ElasticSearch have gone on record before stating that it should never be used as the source of truth.
I'm working on overhauling (rewriting, from scratch) a project right now where the previous developer decided in his infinite wisdom that using ES as a database was a good idea.
I have tried repeatedly to explain to him, and the product owner, why using a search engine as a storage database is a horrific idea. Owner gets it, dev doesn't. I asked that dev if he could overhaul the project, what would he do, and he said "not use SQL at all, and use ES 100%". <_<
I think this is the problem with Mongo too... you have people misusing it, and then want to throw the baby out with the bath water and act like Mongo is the problem, when really, the problem is how they are using it.
There's no need for the vitriol.