A local backup of your most important data is very good to have regardless of any vulnerabilities with cloud solutions. However, with local backups you don't get ready access to your data from a variety of devices remotely. There will always be people who are out there doing malicious things, that's just the nature of the Internet. You can protect your data by ensuring that you enable the MOST security that is available on these services. So using a strong password (use a password manager to manage them) and enabling 2FA if available are all good things..
Services like iCloud, Box, Dropbox, et el are seeking your "business" by providing you a secure way to store your data. When an issue is identified, they have to work hard to ensure your data is safe and not in someone else's hands..but at the same time the cloud services are indeed black boxes to most people. Unless you work there you don't know how they are keeping your data. Is everything encrypted at rest or just scattered on servers? Are they backing up your data in multiple places so that they can restore should someone manage to erase your data?
To a large extent, the cloud is the Wild West of the Internet right now..lots of potential but lots of pitfalls..
