This page: https://docs.mongodb.org/manual/tutorial/backup-small-sharded-cluster-with-mongodump/ contains the following warning: "By default mongodump issue its queries to the non-primary nodes."

However since 3.2 this is no longer the case: when connecting to a mongos or a replica set, mongodump connects to a primary by default (see commit 8fc478259d75235f96ae31fe271bf05f118fa98b linked to TOOLS-250).