Several days ago we upgraded simple Drupal 6 site to Drupal 7. After the core upgrade process was finished we turned on all necessary modules such as Admin menu and Backup & Migrate. Some features required custom update by scripts and some manual work, we made a backup before this activity. ... something went wrong and we go back by restoring DB via Backup&Migrate module and site went down :-( When we took a look on DB backup script we found that there was an empty dump with the structure of some tables. So, as a result, the data in several tables was killed. When we repeated the upgrade process again and took a look at Backup&Migrate advanced settings we were shocked.
By some reason all tables was excluded from backup. After we align B&M settings backup became works correctly :-) The main idea of this post due to automatic upgrade process you should double check settings of contributed modules, upgrade can make them different according initial state. And, of course, backups is necessary part of site development activity but you should be sure that your backups correct.