I believe that the GitHub pull request workflow and issue tracker does not scale to large projects and very much prefer Gerrit/Phabricator. Phabricator was created at Facebook for a large development team (mobile apps) and it shows. They have solutions for issues which only appear at scale (code ownership in monorepos, for example - the owners/herald mechanism). It's also very performant, has almost no dependencies and is extremely easy to deploy.
GitLab is a great GitHub alternative, but it's also copying its flaws.
Great writeup here: (not mine)
http://cramer.io/2014/05/03/on-pull-requests
HN discussion: https://news.ycombinator.com/item?id=7697132
Good quote from that discussion:
> As far as code reviews go, this is pretty spot on. I was part of a a startup that was using GitHub pull requests for code reviews. As the team grew, it became more and more intractable, although not simply because of notifications. Side-by-side diffs and checkpointed diffs (so that you can see what changed since the last round of review and whether/how your comments were addressed) are handled very poorly by GitHub. We ultimately switched to Phabricator, and while there was a little friction as folks got acquainted with the new tool, it made code reviews a much more pleasant process. Recently, I had to go through a full code review back on GH pull requests, and it felt like pulling teeth in comparison. They're fine for interacting with contributors to an open source project, but compared to working with a tool like Phabricator that's built for a code reviewer's workflow (and for teams of engineers working together on a project), they just don't hold a candle, in my opinion.