IIRC one of the problems was github notifies you as soon as a comment is posted, potentially wasting your time if you go look before the comments are finalized. For example you may see a comment asking how something works, and waste time replying only to see the comment was since deleted because the reviewer figured it out.
I don't think the issues were really that serious, but the Go team likely saw no reason to change away from the tool they were comfortable with for a slightly worse tool that they would have to adapt to.
The new GitHub changes allow you to group a set of comments into a "review" that then notifies the recipient all at once, along with letting you mark the PR as "approved" or "needs changes".
Interesting. I found Gerritt's behavior (the flipside) to be a little frustrating. I just have one comment, right here: <x> -- why do I need to publish it and give it a meta-comment?
I don't think the issues were really that serious, but the Go team likely saw no reason to change away from the tool they were comfortable with for a slightly worse tool that they would have to adapt to.