I think a more common occurrence than "let's move everything from MySQL to Mongo", is "we need these user preferences really fast, so let's cache them in Redis"

I think a more common occurrence than "let's move everything from MySQL to Mongo", is "we need these user preferences really fast, so let's cache them in Redis". If you need to do something like that, you can handle all the implementation details in that domain/service layer without changing the API to the rest of your application(s).