It should be obvious to people that the more and higher levels of integration of devices like the Mac and the iWhatever will have some kind of impact on your privacy. The problem with the new handoff feature in OSX Yosemity is that it actually makes a lot of decisions for you. The basic gist of it would be easily described using a comparison with Dropbox.
When using a system like Dropbox the essential core sharing model is “opt-in” meaning that the only files Dropbox gets are the ones that you have chosen to include in your local Dropbox file structure or explicitly uploaded to the Dropbox service. However, when using the Handoff feature it basic will have to upload any files for the compatible apps you are using to the iCloud Drive service in order to then subsequently edit on another iCloud linked device.
The problem here is that this will upload ALL files for these devices and it also won’t ask your permission first it will make the assumption that you will do this and it will therefore need to upload it to Apple servers.
OSX uses a “Saved Application State” in order to power such features as restarting the OS without use intervention and also restarting apps where you left off. All files are stored in this application state structure on disk, this is now also all going to your iCloud Drive if you have handoff enabled. So basically everything is on their servers if you want it there or not.
Hmm … that isn’t exactly private is it.
You can see all this stuff on your hard disk here:
~/Library/Saved Application State/"
You can read a bit more about this here, an article by a security researcher.