> It would obviously need to have great UX

There is the problem. Todo apps are easy to make. However making one that is actually useful for tracking todo items it hard. Getting the todo into the app is harder than writing it on paper. Getting reminders at a useful time is hard (now is not a great time to fix that broken widget - it needs parts not in the budget, I'm at work, it needs a couple hours of dedicated time and I have something else coming up...). I've tried a few different ones, most are a combination of too complex and not complex enough at the same time.