I've used five different code review tools, and Github is the worst of the five. I regularly bug them (Github) about this, and they know it. I hear rumors they've been working on it a lot.
Github's review mechanisms barely scratch the surface of what's possible.
1) Google's current internal one
2) Gerrit (open source, to be used by Go)
3) Google's old one (Mondrian)
4) Rietveld (open source, but run for free at codereview.appspot.com)
I would totally suspect that Phabricator or Review Board would be well above Github (as are 1-4 in my list), but I don't know where. I have little desire to use or explore new code review systems at this point. Four per day is enough for me at the moment.