Data held in cloud storage is generally kept secure. The data in cloud storage servers is encrypted, which makes hacking it a laborious task. The security is not infallible, but breaches tend to come from human errors, like a user giving out their password to a scammer, rather than remote access.
However, it's worth knowing that data in cloud services can be legally viewed. Governments have the right to legally request data that is stored in the cloud, and it is up to the service provider's discretion whether they provide it or not – and they often do. Google, Microsoft and other cloud storage providers get a lot of requests for their data every year, requests they frequently grant. You may not be bothered about the idea of some low-level government employee squinting at your old holiday snaps and "project_final_FINAL2.psd", but if you are, then it's worth being aware that this is a possibility when using cloud storage.