Like others on this thread, I’m using Ulysses, Scrivener, and Final Draft as well as other text apps like Drafts, Simplenote, and Storyist. I’m currently writing three books as well as a LOT of note taking in general and I switch between apps a LOT. I tend to have a mental context switch that goes well with app switches.
In the old days, I’d edit all config files on Unix with vi and do all coding in Emacs. You’d think my brain would fry with the vast differences in command differences, but my brain would just instantly context switch. Unless I thought about it. If I thought about how remarkable it was, I’d mess everything up.
So on iOS/OSX, I have one book in Ulysses, one in Scrivener, one in Storyist. I keep tons of notes in Simplenote, and I do drafts of long form content I’ll end up pasting elsewhere in Drafts. I mostly just dabble in Final Draft - I don’t have any true projects in there yet, unless you count my unlicensed Doctor Who episode I’ve been writing for my kids.