I created an account just to leave a comment here: lack of iCloud sync has finally gotten to me, and I moved both my current projects out of Scrivener. I’m tired of trying to please Dropbox, getting hounded to pay for upgrades and other annoyances that come with that service when I’m already paying for iCloud which works with all my other apps perfectly. It’s too bad your developers can’t find a way to make it work, but others have and after uninstalling Dropbox from my Mac I feel like a weight has been lifted off my shoulders.
Strange… I have been using Dropbox for years and have never been hounded by them to do anything. It just keeps working… for free.
I’ve only had Dropbox for a little longer than a month, but I haven’t seen any hounding to go with the paid service (unlike my free virus program which mostly certain does hound me, and sadly unlike NaNoWriMo which has been after me every since I signed up).
However, everyone has to go with whatever software works best for them. I say this as someone who still uses WordPerfect, although these days not for my writing, which is all in Scriv.
Another happily unhounded longtime (at least a decade) Dropbox user here. I don’t remember ever being nagged to upgrade in all those years. Occasionally they used to send marketing emails a couple times a year, but I must have unsubscribed or something because I haven’t gotten one in ages. The free tier is plenty for all my documents, not just Scrivener projects, which means I don’t have to count doc storage against my iCloud limit. Plus at this point, Dropbox offers more features (like easy sharing) than the simplistic but no doubt ever-improving iCloud, whereas at the moment I can’t think of anything iCloud does that Dropbox doesnt.
It would be nice if Apple eventually opens iCloud to package-type files like Scrivener, but using Dropbox instead of iCloud doesn’t negatively affect my cloud experience at all. And it gives me additional free cloud sync and storage on top of my iCloud allocation. So even if Scrivener did work with iCloud, I would likely continue to use Dropbox, whether with Scrivener or other files, unless there’s some other flaw I’m not aware of.
I’ve never been hounded by Dropbox myself, and I’ve never had any bad service from them - they once restored our entire work Dropbox folder when my son (young at the time) got access to my wife’s computer and dragged some important files to the Trash. 
I’m not really sure what this has got to do with Scrivener 3.1 being incredible, though! I hope you enjoy the software you have moved to and that it works better for you, given how important iCloud seems to be for you. I hope Apple handles it better than their App Store.