Running flatten on a site with eZ Survey extension is use, will result in loss of data collected within these surveys.
Yet, the Known issue text could be improved as followed:
*Please note:* eZ Survey will loose results when archived versions of the survey is deleted. The workaround is to increase the number of archived versions the system will store for survey objects (you need to configure this manually if you have survey classes with custom names (where class identifier != 'survey' ). And never to run flatten or a cronjob that implements it