Hi Sabrina, After looking through a couple of different test instances and the workfront support website, it seems the update roll up hierarchy follows the below:
Object of Original Comment
Object on which the Original Comment Rolls Up
Issue
Project
Task
Project
Project
Program, Portfolio
Program
Portfolio
User
Team
Timesheet
User, Team
Template Task
Template
Story
Iteration, Team
Iteration
Team
It also lists that any document updates would not be displayed in parent tasks or projects. The original design was created to limit the amount of email notifications that are sent out to different users. i.e. a project owner would not receive a notification every time a task update comment is posted. I would recommend to post update comments to individual tasks so the users in charge of those tasks can be notified, but beyond that, it would depend on your users and organizational structure. I hope this helps!
Rob Lawrence Bond Group