If an incremental restore fails early in the operation, the database is still usable (assuming it existed and was consistent before the restore).
If a full restore fails, you do not have a usable database.
If a failure occurs after a certain point in the operation, the restore program marks the database as inconsistent. In this case recovery is only possible by means of a FULL RESTORE. If you were performing a FULL RESTORE when the failure occurred, you may need to go back to the previous FULL BACKUP.