Rather than phrasing this in terms of advantages & disadvantages, I think it might be more fruitful to talk about what problem we would ostensibly be trying to solve by using it.  Right now I don't think the list is high-traffic enough that the big advantage (you can have a discussion without blasting out a message to too many people) would really solve any issues we're currently having.

Personally I am -0 because I don't think that Github's structure for notifications are particularly conducive to an Inbox Zero style approach, i.e. one where everyone involved with he project knows where to go check for things that they might want to pay attention to.  The mailing list makes this very straightforward; you just check your email, which you're probably doing anyway.

Github does have some notion of "inboxes", i.e. https://github.com/twisted/twisted/pulls/review-requested/@me could probably replace https://twisted.reviews, but even though I am a person who thinks way too much about inboxes, I find https://github.com/notifications difficult to approach.  If we were going to turn it on we should probably have a recommended way to check in on messages for contributors.

-g