The other half of what I'd consider "smart sync" is delta sync -- basically only uploading the bit of a file (or other reasonable small chunk) which has changed

The other half of what I'd consider "smart sync" is delta sync -- basically only uploading the bit of a file (or other reasonable small chunk) which has changed. Dropbox does this well, and is very useful if you're only making a small change to a GB file. Last I checked, this has not yet been handled well with encryption. Even worse if you try to do a naïve e2ee cloudstore by e.g. putting an encrypted volume on dropbox.