MongoDB is a really scary database to use at scale.

It doesn't shard nicely. Failovers have rather nasty semantics that can cause nasty bugs in client side code. Performance cliffs abound.

If your datastore is anything over 1TB, I'd be using postgres, or if you can manage it something bigtable-like.

Not always ;). As someone with experience managing mongo at scale, this really speaks volumes to the amount of effort needed to make it not do the wrong thing. And even then, there are unknown unknowns like this that can pop up at any time.