Failures are just a property of distributed systems, some fail in ways that are better for you, some fail in ways that are worse, but sooner or later they all fail. It's a tradeoff which one you choose.
MongoDB was tested in a replicated setup, PG not, that's a pretty fundamental difference. Loosing acked writes in a master-slave replication with failover would be expected with a sufficiently large replication lag even for PG.
MongoDB is about the last database I'd pick for any given scenario, but I acknowledge that there are people that have a usecase where it might be a good fit because the tradeoffs work out for them.
Failures are just a property of distributed systems, some fail in ways that are better for you, some fail in ways that are worse, but sooner or later they all fail. It's a tradeoff which one you choose.