Well… That depends entirely on your threat model…

In my setup, the backup is encrypted locally, and then uploaded to Backblaze. If I leak my encryption key, then yes, Backblaze and any state actor that can compel Backblaze, might be able to read my backup (and the same goes for an encryption vulnerability). But since the connection to access the backup is also authenticated, the rest of the public would not be able to read my backup. If I leak my access credentials, then everyone could get my encrypted backup data, but not be able to decrypt it. Of course if I leak both the access credentials and the encryption key, then yes anyone that obtains both can read my backup.

Many regular people use Microsoft Onedrive or Google Drive, which offers even less protection, but it’s certainly sufficient and well enough protected to keep your dissertation protected.

In most backup services you have the option to choose what gets backed up, and what does not. But sure, it entirely depends on who you want to protect yourself from.

If your main concern is state actors, then yeah… You probably shouldn’t use something like Backblaze. You should keep everything on your own hardware. And convince a friend or some family to have a NAS sitting somewhere that can host your backup destination.

For my case I’m mostly concerned about data continuity (not losing data). But privacy is certainly also a concern, and here I have chosen to believe that the encryption is sound enough, and that my ability to keep my encryption key safe, is sufficient for the data it protects.