I've only used Discussions a few times, and it kinda functions like GH Issues - but I recall somewhere that you can do 1-level nested replies, which I think is a win.
But for me, it's almost entirely down to searchability. Very easy to find stuff via Github, and even leverage Google Search.
However, GDocs are as free-form as it gets, so pros/cons.
Unless there are any substantial or compelling concerns - I can give it a shot, and am very willing to abandon in favour of GDocs if it doesn't "feel right"