How does the WXR file differ from the WPDB?

I'm looking to make it so an administrator can do a complete backup of a WordPress website with the click of a button in the administration pages. In doing so, I figure the code would need to backup /wp-content for its themes, plugins, and uploads directories, as well as do a database backup. But backing up the database could be tedious, especially with privileges issues.

So, why not use WordPress's own WXR file? According to WordPress, the WXR file backs up the following information:

This format, which is called and WordPress eXtended RSS or WXR file, will contain your posts, pages, custom post types, comments, custom fields, categories, tags, custom taxonomies, and users.

How does the WXR file differ from the WPDB? That is, what would I be missing if I don't back up the WPDB and only went with the WXR file? Could I achieve a complete WordPress backup via the manner described above?