For anything I see as really important these days, I avoid anything that is not open source. It used to be idealism, but now I see it as practicality. If something happens to Atlassian, I am up a creek. I will call this the Perforce Scenario (for the resulting problem, not the originating issue), because Perforce revoked its exception to its proprietary SCM when a Linux contributor reverse engineered it for an open source client he wanted to use with it, violating the terms.

So Atlassian might be free for open source projects (they say for Stash at least after Googling), but that can always change with any app or service you use. RMS free should not change, and you should be able to understand the most current copy you have of source to find alternatives or build your own. Fortunately, this rarely happens for the core FOSS items I use. I have been lucky.

Also, Atlassian can be pretty hefty for small weekend projects in my opinion. But I have only used Confluence at work, it seems like it would require much more resources than a server or VPS for weekend projects would require. I could be wrong.