Disclaimer: I used to work for MongoDB.

I think that MQL is very difficult to write because of its verbosity—it simply requires so much text. Even as someone who regularly wrote and debugged MQL queries, it is just so verbose, but essentially is just an AST structure and doesn’t need that much verbosity. I created a library that attempts to solve that friction, but I have no idea if it’s still being maintained by MongoDB: https://github.com/mongodb-labs/pymongoagg