In larger organizations, it becomes a bit harder to browse activity based on updated pull requests, and extremely large activity feeds. The problems are as follows:
New PR experience removes the Activity feed tab. Large organizations sometimes depend on this for updates.
In large pull requests, the truncated activity feed often will miss something that came in via e-mail. If someone commented on the diff view, then this comment will be lost by the time the pull request gets updated. Going to the affected file in the new right pane doesn't seem to have any comments associated with it.