Okay, I wasn’t aware that Scrivener on macOS is so much more advanced that even multi-user collaboration is possible.
Well the method we are using is not too crazy. Don’t mistake what I’m saying for the tech that companies like Google make, where everyone can see each other’s cursor and stuff like that. It’s more along the lines of an internal sync system that can take a project that someone has been working on, and sync its changes into your own. So in this way you can send out copies, everyone works at their leisure, collect them, sync them and then send out updates and repeat. That is why any collaboration group only needs one Mac user in it.
And it shouldn’t even be too difficult for us to get this caught up in the future. We based much of the internal logic for it, on the Mac, using the same internal syncing code that it uses to merge mobile edits (from the iOS version) back into the main project. The problems and their solutions are very similar given how the iOS version works on a internal mirror of the project instead of the original directly.
I connected Scrivener to Dropbox because, at the time, this was either recommended or the only officially supported option. I don’t remember the exact details anymore.
Oh okay, yeah that’s one of those myths I was referring to above. 
That’s never been true, but for a while (maybe fifteen years ago), it was certainly one of the most reliable services and the founder of the entire genre, so a lot of the talking up about it in that fashion has over the years, and over many shared telephones, turned into exaggerations. At this point I would say it is, if anything, a mediocre middle-of-the-pack option. One can do much better.
But yeah, again I would encourage searching the forum for conversations on the wisdom of having us encrypt your data for you. This is something that has been discussed forward and backward at this point. I’ll provide a link to one such thread below.
When I log into my Dropbox account now, I can browse the folder, click on a file like content.rtf, open it with a text editor, and read my text. It may be unformatted, but it is clearly readable.
To be clear, I wouldn’t trust Dropbox any further than I can toss the headquarters for the NSA, but I don’t think that is evidence of anything in and of itself. It is entirely possible for a service to provide end-to-end encryption not only with their client and your connected devices, but within a website too.
Here are some threads of interest: