I'm not an expert on this matter but I think it's something like; once the FAT table on a "disk" has been corrupted you are in trouble, chances are quite big that you will not be able recover all data. One of the problem is that files might be fragmented so when a recovery program searches for deleted files it might find the beginning of the file, but could fail to find the rest of the file, leaving you with a defect file. Files which is not fragmented is "easier" to recovered.
Mostly memory cards are destroyed when unplugging it; with power on, or while files are still written to the card, if a cache is enabled and the card is unplugged before the cache is flushed to the card and by electrical shocks. So when taking the card out of the camera, make sure to switch it off - I have notice that when I switch mine off, the green led is lit so the camera might be reading from or writing back to the card, either way the card is under power.