What is a potential drawback of using only differential backups?

Using only differential backups can indeed lead to longer restoration times if multiple backups are necessary for recovery. The reason for this is that differential backups save the changes made to the data since the last full backup. As time goes on, each differential backup will grow in size, since they encompass all changes made since the last full backup. When a system needs to be restored, it requires the last full backup and the most recent differential backup. If many things have changed between the full backup and the current state, this could mean that the size of the differential backup has increased significantly, requiring more time to restore all of that data.

This restoration process can be far more time-consuming compared to incremental backups, which only contain changes made since the last backup, regardless of whether that was a full or another incremental backup. While differential backups can simplify the backup process since only two backup sets are needed for recovery (the last full and the last differential), the cumulative nature of the differential backups can lead to a more extended duration for restoring the data if a full restore is needed and many changes have occurred since the last full backup.