This app allows you to link your Dropbox account for syncing to multiple devices, but there’s no iCloud support. It can also export Markdown or HTML files, so that’s more end-to-end solution compared with other writing apps.
Writing Kit comes with a “Quick Research” tool, a search engine built into the app that gives you a quick way to look up facts or perform calculations.
I'm a sucker for articles on iPad writing apps. The above article gives you a few to try out.
Writing Kit's excellent.