One important thing to note is that all your examples of custom text are non-rich editors. Text editing becomes astonishingly more difficult if proper rich text

One important thing to note is that all your examples of custom text are non-rich editors. Text editing becomes astonishingly more difficult if proper rich text and i18n is required. Then, Core Text / TextKit is a pretty good solution because anything else quickly requires years of work. The best alternative I know of is using a Javascript-based editor in a WebView - or jumping to something like Qt.