I want to copy a wiki to a different course. I tried with backup and restore, yet the restored wiki was completely empty except the description text field.
I guess that's a feature. Same will happen with a Forum, Glossary or Database activity.
This has to do with the fact that the content in these activities is generally created by your participants, making this 'user data'. If you backup/restored the activity or course without the enrolled users, the user data will not be incorporated in the backup file.
You can, though, anonymize the user data, and then create the backup file as a workaround.