Realistically, by the time you upload your work to the cloud, security is out of your hands. Google and the like have good security measures in place, but breaches do happen from time to time. On top of that, a breach may happen to your account specifically, in which case, your little attacker now has full access to your work. Regardless of the consequences, you really don't want that.
The current AI craze is also an issue. Some of these cloud companies (like Google) have come out to say that they won't use your data to train models. But their word isn't absolute. In the end, they have their own interests, and they are massive enough to abuse their power from time to time. Facebook survived the Cambridge Analytica scandal, and also the "Facebook Files" case, which was just shocking.
Encryption is a good defense. Encrypt your work before you upload to the cloud, and decrypt it locally. Doing this manually is a pain but there are software to automate the process. But that software should also be trustworthy.
The way I deal with it currently is with a bash script. It periodically creates encrypted 7zip archives of my writing directory and uploads them to OneDrive.
I'm also currently working on a C++/Qt novel writing app that will seamlessly sync writing projects to OneDrive with full encryption. It's my university thesis project. No, I don't plan to release it publicly—it'll be private only so I can customize it to my heart's content and make it my dream writing app. The automatic cloud encryption/decryption feature is one of the primary things I look forward to.