Loading data into the target is often the area where problems occur because it’s ultimately a verification that the previous steps were correct.  In some cases, failure may mean that an error occurred when attempting to migrate the item, in other situations an item may migrate to the target successfully but there are issues with the item.

‘Failure examples’

• Item is migrated to the wrong location due to Structure mapping
• Item may incomplete or incorrect in the target due to content type/metadata mapping issues
• Incorrect target users or groups specified in security mappings

Since Load is the stage where content is retrieved and copied into the target, it’s obviously and area where performance issues can arise.

For information on SharePoint best practices see https://docs.microsoft.com/en-us/sharepointmigration/sharepoint-online-and-onedrive-migration-speed