I should also point out that sandstorm only gives apps a single mounted directory that is writable and every "grain" (running instance of an app) has a download

I should also point out that sandstorm only gives apps a single mounted directory that is writable and every "grain" (running instance of an app) has a download button that includes that mount point, so while sandstorm doesn't dictate data formats, you can get your data out and likely migrate it to another app.