SSD are pretty safe in general. Amazon also makes replicate of your data. It's the same if not safer than other NoSQL solution. Definitely safer than those RAM-based NoSQL approaches.
The "back up to s3" part is for archiving data, like periodic snapshot backup of your data so that you can get back the old data in case they are deleted by app/human.
The "back up to s3" part is for archiving data, like periodic snapshot backup of your data so that you can get back the old data in case they are deleted by app/human.