GitHub does have the ability to assign to (
https://help.github.com/articles/assigning-issues-and-pull-r...) and filter by assignee (
https://help.github.com/articles/filtering-issues-and-pull-r...). If you want to pull in multiple reviewers, you can @mention them.
Even in Phabricator, though, when you have multiple reviewers only one has to accept for it to be accepted. Oftentimes it is _more_ confusing to have multiple reviewers because you don't know who should have the final say.
Github has Hub (https://github.com/github/hub) which is as powerful, if not more so, than arc.
The commenting system is a matter of preference, but having forgotten to "Clowncopterize" (a great demonstration of the professionalism of those who hack on Phabricator) comments, and given that they thread in email, it hasn't been a problem for me. Though I understand where you're coming from.