Well, that’s only if one absolutely constrains themselves to the subjectively most convenient path and refuses to use the plethora of other methods available. The iOS version supports many ways of working with your data that do not involve Dropbox, some as about as secure as an iOS device can get as they involve plugging the device physically into the Mac/PC and copying data around like you would to an external drive. Other methods are more convenient than USB cables, and are as secure as your WiFi network is, such as AirDrop and WiFi disk management (which you need to enable on the desktop last I checked). Other methods are even more convenient/fault-tolerant in that you can make use of whatever cloud service you consider safe, bringing back the ability to forget to update your device before you leave the building.
The user manual goes over all of these methods in §14.2.1, in the section on working with the iOS version of Scrivener, and there is also a more extended guide on the cloud-based method mentioned above, as well as a discussion on the forum that goes into the merits of this method in-depth, such as how rock solid it is in protecting us from:
- Machine errors in copying data around over the Internet.
- Human errors in the use of such technologies.
More salient to the topic at hand though, they grant us the power to choose whatever service (or technology we cook up in a closet server) we want to use. I use Tresorit myself, which is backed not only by EU privacy laws, but even more strict Swiss law—and they don’t have my decryption key anyway.
I wouldn’t ever expect Scrivener (or any program) to support niche services like that, but that’s okay because the method I describe above is in my opinion a better and safer way of working than having several devices all manipulating “one” data pool anyway.
It’s just… a little less convenient. 
But, I don’t cook with a microwave either, so I’m not bothered by that.