Based on findings, there are still a lot of issues with mongodb as persistence layer for AEM 6. Few points to be noted:
- "Sharding" feature of Mongodb is not supported by AEM yet. So horizontal scaling of Mongodb Primary is not possible yet to use with AEM.
- Performance of AEM with mongodb is not as good as compared to TarMK.
- TarMK is the best practice still by Adobe support team. AEM replication can be used to sync content from one data center to other to have DR center ready with TarMK and publish content to multiple publish instances.