The problem is that people seem to focus on some tiny aspect that it might improve, like "how easy it is to deal with data of varying shape", ignoring the fact that by ignoring the problem, you aren't making it go away.
You always need to know what kind of data you are dealing with. If you use mongodb, you no longer have the database as the source of truth for the shape of your data, so you need to maintain this somewhere else, whether it be in mongoose models, in your API layer or just in your frontend. The point is that you aren't solving a problem as much as pushing it around, and usually pushing it around results in it being handled in the wrong place(like the frontend).
