Full also wipes out any previous customization you may have done with tweaking descriptions, categories, etc on your websites backend.
Scenario: You setup your website with Full and give it a few days to load all the data. A week later you notice some meta data is not to your liking (i.e. some colors are wrong for products). You decide to customize the meta data on your websites back end because you are already looking at it and it would be easy and quick.
A month later you decide to change your catalog structure through inventorysource. You made such large changes in the structure of your product categories the quickest way to have it synced is to push a Full update. When you do this, any custom updates (such as the color changes above, etc) will be LOST!