We tried to do a migration from Sql Server to Postgres.

Even went as far as building a parser/compiler to transform Sql. Certain things are just too difficult to handle.

1. Sql Server supports arbitrary sql statements in queries. Postgres does not.

2. Data types are not the same.

3. Feature sets at the 'interface edge' between the client library and the engine differ. For example, Sql Server queries can return an arbitrary number of results.

It's true that Mongo uses a custom language, but with Mongo - you are unlikely to have any significant business logic in your data layer.

Also their API surface is relatively simple.

For example, FoundationDB has a layer that mimics the Mongo API. Amazon has one as well.