I'd have thought the key vulnerability would have been in transmission, not data storage.
The main problems are security and privacy. Anything that you allow into the cloud should be regarded as being placed in the public domain and open for analysis for the advertising industry, law enforcement, etc. Nobody is giving you free storage out of kindness, you are not the customer, you are the product.
You are also trusting the storage provider to stay in business and continue their existing product strategy forever.