When your data is stored online it is distributed at more locations increasing the risk of unauthorized physical access to the data. By sharing storage and networks with many other users/customers it is possible for other customers to access your data.
Outsourcing data storage to another company needs careful investigation and nothing is ever certain as the companies are not permanent and the services and products they provide can change.
Reliability and availability depends on wide area network availability and on the level of precautions taken by the service provider. Reliability should be based on hardware as well as various algorithms used.
Security of stored data and data in transit may be a concern when storing sensitive data at a cloud storage provider. The cloud holds data from many different users and organizations, hackers see it as a very valuable target.