A course backup (.mbz) is probably only as good as the DB backup. By that I mean, if the DB backup was loosing assignments, etc., then there is a good chance that the course backups don't include that information.
However, having said that, one way to know for sure. Download a 'typical' course .mbz backup to a local HD 'test' folder. Rename the file extension to .zip. Unzip it. The resulting folder will have folders and .xml files. Inspecting the .xml files with a text editor will show tags that reference activities ... such as assignments ... if the backups contain that information. There will also be folders - like course_files. One will have to use the files.xml file to find the location and hashed name of files contained in course_files.
Best of luck!
'spirit of sharing', Ken