I just purchased Storyist for iOS today, am using it on my iPad Air 2 and iPhone 6s. Both work fine, sync well, no problems. I can go my iCloud Drive folder on either one and find the Storyist file folder, along with the files I've put there.
However, when I go to my iCloud drive on my MacBook Pro, I cannot see the Storyist folder. I've tried going to iCloud drive via Finder, and also through Safari, logging in to my iCloud account. I've logged out and back in multiple times; started and re-started my MacBook multiple times; and added an item several times in an attempt to "force" a sync. But none of these have worked. I can still see and access the Storyist folder on my iOS devices; I still cannot see it nor access it on my MacBook. (Note - on my MacBook, I am using the trial version of Storyist - I'm in the first day of the 15-day trial. I was trying to decide whether to move to Storyist since Scrivener still does not have an iOS version. So far, I love what I see, but obviously I want and need iCloud sync functionality.)