Writing good spaced repetition memory prompts is hard, but I suspect it’ll be even harder for authors to write good prompts for other people, as they must do in the Mnemonic medium.
When you write a prompt for yourself, there’s at least some chance you’ll notice problems during your review sessions: you might notice that you always get a prompt wrong, or that it produces a sigh. But authors are presumably not learning this material anew themselves, so they either won’t review the prompts themselves, or if they do they’ll be less emotionally attached. (See To what extent do review sessions offer prompt-writing feedback? for more)
Authors will also suffer from the Curse of knowledge, which will tend to make them believe that their prompts are working better than they really are.