My opinion is that nothing that currently exists really solves the whole problem of information management well (aside from git). Most teams mash together three or four different solutions for strongly related problems. Some teams are as bad as: gitolite for central code repo, a bug tracker for managing issues, a spreadsheet for managing who works on what features, a separate bug tracker for issues found in QA, personally-managed to-do lists for each developer, a Trello, a dozen bespoke reports for status to upper management, and all team communication goes through e-mail. Yes, slack and github would replace about 3 of these, but you still have 5 solutions to the same general problem -- what needs to be done and who is doing (or will do) it, how, and when.
Obviously this is a very hard problem, or it would be solved now. I have tried for a long time to decide what I think would be a good solution, and for all intents and porpoises, I have gotten nowhere.
I really like the approach Trello has taken -- provide a simple, flexible concept and let users build it all from there. Obviously Trello doesn't tick all the boxes, but I wonder if some other deceptively simple idea can accomplish it.
There are other alternatives with pretty decent ecosystems if you don't want to pay the $10 dollars to get 10 seats for Atlassian (which again, I think youd be crazy not to at least demo it). RE: OSS - Even with a "crappy" 8 year old Redmine install, you're given a git master remote to push to, a document share, bug tracking, and a whole lot more out of the box. GitLab also has a really integrated free set of tools which is a huge huge RAM hog in itself, but it's free and has eye candy so you can probably get management to sign off.