I’ve had this discussion before with L&L, I doubt that the technical limitations (in iCloud) that they always mention have been solved since.
In short, Scrivener sync works without issue… provided that you press sync and remember to fully close the project, and give time for the changes to be uploaded, on each device, before you dare to open the project on the other device. This was a cutting edge user experience in 2006, but for years now it has not been how I (and many others) work in the real world.
In the real world, I’m typing on my iPad, then I may have to slap it shut quickly (say, my friend has arrived at the coffee shop, or my cab arrived, or whatever), and I may want to check on something in the project on my iMac that evening. The iPad is maybe still in my bag, or maybe I left it at work. Or I may even dare to have the project open on two devices at the same time! Perhaps accidentally, I forgot it close it on my iMac this morning and I’ve since left home, now I’m out and want to keep working on that project.
My point is just that the dropbox sync works if you are the textbook user, who is sitting down for a session of work, and then closing everything diligently, etc. Real life (mine at least) is way messier than that. Whenever I try to use dropbox and scrivener, I am soon in a never-ending loop of yellow conflict reconciliation pages, and it all just feels so slow and precarious.
Simpler apps than Scrivener can cope with this alright. In some you can have the same document open in many places simultaneously and watch it update in real time as you type on one of those devices. But of course, those are simple word editors I’m talking about, and Scrivener is a more complex beast.
For me, ultimately I prefer to sacrifice some functionality to have a synced app that I can rely on. But I know I’m in the minority here, of course