I don’t trust any storage that I don’t adminsiter for private data, which is why I currently rely on Mega, which encrypts files on your machine locally BEFORE transferring it. Kim Dotcom (founder of Mega), however, has mentioned that Mega will not be in service for much longer and that users should back up their data in preparation for potential shutdown. Without Day One providing the option to store data in a custom directory I couldn’t use this approach but it’s airtight in terms of privacy and security. I don’t worry about encrypting files locally as a separate concern, because if someone has that kind of access to my system the point is fairly moot anyhow.
If Mega no longer becomes available I’ll most likely turn to a git-based sync solution.