Earlier quoted context omitted.
I very strongly prefer, as an engineer, not to use Atlassian products (Jira, Hipchat, Bitbucket, etc).
okay, i'll bite. My employer recently made the switch from Hipchat to Slack, and I can't really tell any difference in functionality. The only thing slack has is that it's pretty/shiny, which doesn't really make any difference to me personally. If anything, I miss being able to arrange the channels like I could in Hipchat. Also, what does Github give you over Gitlab?
HipChat is playing catchup constantly with Slack (except the @here feature; Slack was behind the ball on that one). Slack's channel integrations are incredibly smooth. Message delivery is so much more reliable on Slack mobile than HipChat's app ever was (perhaps this was iOS specific?). Message management (stars, pinning, history links I can take from Slack and throw into Github issues, commits, or other SaaS team tools). The management of multiple teams in one interface on both mobile and mac desktop app (Slack handles this extremely well).
Github issues are enough for me to stick with GH. It Just Works. So you don't have to provide me with Github, Slack, etc as an employer. But it'll effect my decision to work there. I've interviewed people at my previous job, and they have flat out decided not to join when they were told we weren't using Github and had no plans to move to it.
When you're spending 8 hours (or more!) a day in tools, you expect them to be the best/easiest/most productive to use.