mongosync is not intended for bidirectional synchronisation.
Cluster-to-Cluster Sync provides continuous data synchronization or a one-time data migration between MongoDB clusters. For notes and caveats on continuous sync, see the Considerations for Continuous Sync page. You can enable Cluster-to-Cluster Sync with the mongosync utility.
The second item in the general limitations list specifies “The destination cluster must be empty”
Please ensure that your application is not affected by the limitations. Running
mongosyncin the presence of one of these limitations could lead to undefined behavior on the destination cluster.
If all the Considerations and Limitations are assessed and followed there will be little chance of issues.
refs:
https://www.mongodb.com/docs/cluster-to-cluster-sync/current/reference/mongosync/mongosync-behavior/#std-label-mongosync-considerations
https://www.mongodb.com/docs/cluster-to-cluster-sync/current/reference/limitations/