I understand what you mean by serious development, large complex projects youre working on for an extended period of time, but that's not all development. For example a few years ago I needed to do some coding exercises for a job application and I did them all on the train during my commute on my iPad in Pythonista. There are now several excellent coding apps, git clients, decent editors etc available for iOS you can even use to develop apps for publication on the App Store.

Until the iPhone 6+ I'd have said coding on a phone was really awkward, but nowadays on the modern large format phones it's quite workable.