You are correct that, from a security standpoint, your software is no different than any other software I install on my computer, since desktop computers have no sandboxing. But from a privacy standpoint, it could be uniquely concerning.

With Google Drive, I choose which files to upload. It doesn't have broad access to everything on my computer.

Dropbox, iCloud, and OneDrive are just backup services, so in theory they could just back up your files as an encrypted blob and have no way to read them. Unfortunately, they don't encrypt them (which is partly why I don't use those services). But at least I have their "promise" that they won't read or analyze my files, which would make me feel better even if its a weak promise.

On the other hand, your service, by nature, is reading an analyzing all of my files using a remote server.