(Jason Scott is speaking from experience here, as one of the people called in to do emergency archive response when these cloud businesses shut down.)
Edit: jacquesm points out the sudden death of cloudspaces.com, which I'd forgotten about: http://www.infoworld.com/article/2608076/data-center/murder-...
A single computer security problem (which happen on a daily basis, although not usually at this severity) could enable your cloud to be deleted.
The "counterparty risk" is small but ineradicable. The finance industry re-learnt this with Bear Stearns recently.