In general using maniphest tasks for a "story" --> "task" level of granularity feels right (we certainly don't want to make a project for each story and then have 10,000 projects a year from now) but from the point of view of an engineering lead (or someone else who isn't looking at the task everyday) it is difficult to figure out "What is going on?" with the subtasks without opening all of them. The only way that subtask state is reflected in the parent is whether it is opened or closed.

Engineering lead questions:

• Do these subtasks depend on other engineering groups? (Display the projects for each subtask? Include projects/workboard-column in hovercards?)
• Are at least some of these open tasks making some sort of reasonable forward progress from moving around on workboards? (Display workboard column as above? Display something else that could be triggered via T5474?)
• If there are "a bunch" of subtasks, which ones are important? (Display priority somehow? Natural sort instead of just by id?)