I definitely agree with this (disclosure: I'm Gitcolony's founder). I think that PRs are a great start point but not the best way to review code in a company. I do believe they are great option for open source contributions but not the most efficient one for companies.

+1 on Piotrs comment (disclosure: founding team of ReviewNinja - http://review.ninja). We also found that most users like to put some more structure around their code review and appreciate the fact that tools like the ones mentioned by Piotr give more visibility into outstanding issues that need to be closed, before a pull request can be merged. It is also helpful for most to get a better handling around who 'approved' (+1ed) a PR.