I have used Byword for all my long form writing for years now. It’s an excellent plain text editor with a beautiful, minimal interface. Unfortunately it has relatively little development support and I’ve been noticing bugs, particularly with iCloud sync with the iOS apps. Byword’s major benefits are it’s interface and the fact that it saves all your files as plain .txt files in iCloud so they’re compatible with virtually any other word processing software.
Because of the bugs however I’ve been considering switching to something else, and I think I’ve finally made the decision to move my long form writing to Bear. Bear is a little less simple but supports markdown and does have a relatively minimal interface. Although it’s a little more feature rich than Byword much of the interface can be hidden while you’re working. It’s developers are active and regularly updating and improving the app. It implements a lot of optional iOS features such as alternative home screen icons and has more powerful tagging and search functionality. It even has an Apple Watch app, which allows you to read your notes if you really want to and also dictate new notes if you need to record something in a pinch.
The one downside is that your documents are stored in an iCloud hosted SQL database, and are only accessible from within the app and not via the file system. It is possible to export and backup your documents however so they aren’t stuck in the app if for some reason development ceases. You’ll want to make sure that you backup fairly frequently if loss of data is a worry.