I’m with Dr. Drang here. I’ve tried many of these apps and they just never stuck with the kind of writer I am: Drang’s “kind that constantly fiddles with the text”.
I am constantly referring to photographs, drawings, experimental test results, calculations, reports written by others, textbooks, journal articles, and so on. These are not distractions; they are essential to the writing process.
Replace that with other people’s blog posts, benchmarks, and Apple Store configurators, and that’s me.
I think this is also why I’ve always found writing on iOS painful and ultimately not worth doing for me.