I feel like MongoDB/NoSQL is a horse that's been beaten so much in the last few years that no one is actually making that choice nowadays. Hasn't everyone already learned to stick with Postgres?
Unfortunately, no. Some friends of mine picked another nonrel database (not Mongo) for their new company, which prompted this article. I still have to give this advice quite often.
Blindly picking a SQL DB (mysql/postgres etc.) is quite expensive from the get-go (a production ready mysql/postgres would cost ~30$/m).
Mongo costs ~$10/m (MongoDB Atlas), Google's Datastore is Pay as you Go (so your initial cost is close to $0 till you get paying customers), AWS's DynamoDB is similarly priced as well.
Sure, sadly all those noSQL solutions get really expensive as your usage goes up to normal non-webscale proportions, but at that point you have the $ to invest in a SQL solution.
The above was mentioned with bootstrapped startups/services in mind. Not your usual million funded valley companies.