Flarum is pretty stable (as you can see on this site). It's not quite mature enough, although we're quickly getting to that point. We still don't recommend Flarum for production sites, however many people are using it for live sites regardless.
Note that with the recently released extensions, I consider Flarum nearly at feature parity with other forum softwares. There's a few places were it could mature some more (user management, in particular), but for a small forum it should be fine as long as you can accept some risk when upgrading.
