Both dropbox and icloud are the same on the security scale - the format is exactly the same. The "safest" would of course be a local sync through local wifi, but it has it's usability problems. Then there's 1passwords "for teams" cloud system, which I use, and the paper is avaible, but I haven't read it yet, so can't vouch for that.
That said, opvault is pretty ok security wise.
That said, I discourage the use of dropbox, since they plan on completely bypassing the VFS layer in the system and writing their own kernel driver for dropbox, making it mandatory. It's theoretically for seamless streaming, but *that* is the part that's highly unsecure. I don't know if they implemented it yet, but just the audacity tells me they don't give a crap and both stability and security. The moment I read this (and the highly critical comments which they chose to ignore) i deleted my dropbox account and migrated to a self-hosted nextcloud.
Source : am a mathematician and have dabbled in theoretical crypto on more than one occasion
