ES grew out of Lucene, which provided an inverted index of all the text in a document, with a bunch of NLP related features bolted onto that. While ostensibly designed to be developer friendly, ES had and still has a horribly hacked together API with bugs and mis-documented misfeatures all over the place. In my experience it's anything but developer friendly. But if all you need is a text index on a document store with a static schema, it does its job reasonably well.
Mongo started as a very developer-friendly data store with lots of overstated claims to being a database. While earlier versions of Mongo were wildly dangerous to use as a business critical database, it has since matured and is now quite good at being a developer-friendly document-centric database. In my experience Mongo truly is developer-friendly as long as you don't try to use it as a full-blown transactional database with lots of complex data shapes and indexes.
I would not trust ES with anything but text search on a document store, and I would not trust Mongo with anything resembling multi-document transactions. With that said, they are both good at specific, different things.
MySQL and Postgres have their own baggage that makes them pretty terrible in some aspects. IMHO a JSON-over-HTTP API should really be table stakes for a database to be considered developer-friendly nowadays. (But please don't butcher HTTP like ES did and then claim to have that.)