There's very little good use cases for mongo, change my mind.

Prototyping maybe? Rails can prototype, create/update/destroy db schemas really quickly anyways.

If you're doing a web app, there's tons of libs that let you have a store in your app, even a fake mongo on the browser.

Are the reads fast? When I need that, use with redis.

Can it be an actual replacement for an app's db? No. Safety mechanisms that relational dbs have are pretty much must haves for a production level app.
Data type checks, null checks, foreign key checks, query checks.
All this robustness, this safety is something critical to maintain the data of an app sane.

Screw ups in the app layer affecting the data are a lot less visible and don't get noticed immediately (things like this can happen with relational dbs but are a lot less likely)

Let's not even get into mutating structures. Once you pick a structure with mongo, you're pretty much set.
Redoing a structure is manual, and you better have checks afterwards.

But at the same time, this is kind of a pro for mongo, since if there's variable data, as in some fields that are not always present, you don't need to create column for them, they just go into the data.

But you can have json columns in postgres too!

Is it easier to migrate than relational dbs? yes, but docker makes everything easy also.