The main advantage is that those forums would be under the direct control of the developers themselves.
No they won't. They will be under the control of Github. Good luck trying to reach out to them if you have to deal with a spammer/bot, or a troublesome user. And pray that Github doesn't discontinue them 6 months from now, like they did a few years back with download hosting. I used to host my downloads there, then they removed the feature, forcing me to move to Mediafire. They added the Release feature a few years later, but that was nowhere as flexible as the original download feature.