Its scalable in terms that if your total data doesn't fit into single machine, you can distribute it between shards in mongo, but can't in pgsql. replica set i

Its scalable in terms that if your total data doesn't fit into single machine, you can distribute it between shards in mongo, but can't in pgsql. replica set is for redundancy and availability, but you can use replicas for reads, so scale your reads traffic. pgsql works absolutely the same way, you have one master which accepts writes and read-only slaves/replicas.