Ranked in these QuestionsQuestion Ranking
Pro Can reference issue numbers and commits in comments
Because GitHub pull requests are integrated with the rest of GitHub, you can reference parts of your git history in comments. This can be handy if you want to point someone to the historical significance of a piece of code.
Pro Already integrated with GitHub projects
Pro Free for Public/Open Source Projects
Con Not free for non-open source code
Con No "acknowledgment" or "mark as reviewed"
The code review process should enforce the code to be reviewed by one or two peers at least to be "accepted" or "reviewed". On github, there is no such thing : no way to be sure that a PR has been reviewed by a certain number of reviewers.