This is also because I think of mongo as generalist, which may or may not be right.
It seems as though there are better choices for more specific use cases
It's not like you couldn't rip out mongo in MERN and use PERN or MyERN (Mysql Express React Node). There's some good libs/packages for using relational db's, and the benefits may outweigh those of Mongo.
I guess one other use case maybe would be an incremental/idle game, where all data is just stored as one big json doc, and you just need to connect/update totals, then sync that data back and forth, with not a lot of relationship/connections or transactional data.