Understanding Cloud Backups/Sync Services
You should not trust cloud providers with your (not previously and locally encrypted) sensitive data and you should be wary of their privacy claims. In most cases, they can access your data and provide it to a third party if they want to.
Examples
"Media and messages you back up aren’t protected by WhatsApp end-to-end encryption while in Google Drive.".
"Messages in iCloud also uses end-to-end encryption. If you have iCloud Backup turned on, your backup includes a copy of the key protecting your Messages. This ensures you can recover your Messages if you lose access to iCloud Keychain and your trusted devices.".
"When you use OneDrive, we collect data about your usage of the service, as well as the content you store, to provide, improve, and protect the services. Examples include indexing the contents of your OneDrive documents so that you can search for them later and using location information to enable you to search for photos based on where the photo was taken".
"To provide these and other features, Dropbox accesses, stores, and scans Your Stuff. You give us permission to do those things, and this permission extends to our affiliates and trusted third parties we work with".
The only way to mitigate this is to encrypt your data on your side and then only upload it to our recommended services or just not use them at all.