Is it safely or not to switch between import from AD DS option and synchronization option in order to quickly upload the missing contacts?
When you intend to switch, you should be aware of
- Each time you change the sync mechanism from
SharePoint Profile Synchronizationto
Active Directory Importthe user
profile synchronization serviceis stopped. You will need to restart it if you have switched again to
SharePoint Profile Synchronization.
Advantage to Switch
- Extremely fast performance than SharePoint Profile Synchronization.
- Very reliable
- Connect to forests with multiple domains
- Windows, FBA and claims are all supported
- Cannot import from any other user repository than AD (no LDAP support)
- Sync is one way from AD into SharePoint
For more Details Check Active Directory Import for SharePoint 2013