Hi,
Katherine has given lots of good answers here, so I’m probably not adding much, but:
Basically, it’s perfectly safe to store your files on iCloud Drive, nearly as much as it is on Dropbox. However, with any cloud service, you just have to be careful to make sure that files are fully synced between machines before you use them. So, if you edit a project on computer A, you need to be sure that the changes have uploaded to the cloud service from machine A and also downloaded to computer B before you use the project on computer B. And you need to have the project closed on the computer on which you are not using it to avoid conflicts.
As long as you follow these rules, you will be safe on iCloud Drive or Dropbox.
The downside of iCloud is that it is not as transparent about when it syncs files as Dropbox. Dropbox has an indicator right in the menu bar showing when it is syncing. As soon as you make a change to a file in Dropbox, it starts uploading. And you can see the sync indicator on your other computer to check that things are downloading. For iCloud, you have to find the file in the Finder, because the upload/download progress is shown there, not in an easy-to-see place such as the binder.
Also, Dropbox is much better at making conflicted file copies than iCloud Drive. If you do accidentally work on a project that hasn’t finished syncing, Dropbox will make file conflict copies, and Scrivener can retrieve them. You’ll then get a message in Scrivener telling you that unexpected files were found and you’ll have to decide how to handle them - an annoyance, to be sure, but at least you can recover your work. On iCloud, conflicts are just lost, so you could lose work if you aren’t careful about following the rules of ensuring that everything is synced before you open and work on a project.
In most apps, there are no real complications, because most apps have a file format that consists of a single file per document/project. But a Scrivener project is really a folder containing many files - thus allowing the user to bring in research files, and allowing Scrivener to run fast even for massive projects, because it only has to load into memory the files it needs for the current session (whereas single-file apps have to load the entire file into memory). So Scrivener is essentially a hybrid between a shoebox app (a UI for a repository of many files, such as Photos) and a document-based app (an app in which each window operates on a single file, such as Pages). iCloud wasn’t built with this sort of app in mind, because most apps are one or the other rather than a hybrid. This is why you have to be a little more careful in Scrivener.
Ultimately, though, I would just say: if things are working for you on iCloud Drive, just carry on. Just be sure to have Scrivener’s automatic backups feature turned on, so that if anything does ever go wrong, you can easily restore from a backup.
All the best,
Keith