Pick Milestones if
- Your work has phases, not just due dates. You want a real 'active milestone', not a flagged task.
- You want to mark a project as an idea or as abandoned without losing it.
- Reminders' single-list-per-project shape feels too flat for what you're actually planning.
- You want a dedicated planner app, separate from your grocery list and your 'pick up dry cleaning' nudges.
- You want a calmer surface that isn't also receiving Siri-dictated reminders all day.