There are already numerous active communities that are using Flarum despite it being in the beta stages. If you did a quick search of the forum you will find that there are plenty of topics that have already addressed this question numerous times and some will even link you to some really good-looking live instances.
At the end of the day a forum is as stable as you make it. On my Flarum forum, I do not use extensions or anything that could potentially spell disaster for any future releases or the stable release (I am using the default theme with no customisations either). May be it's more for peace of mind but I'm not a coding guru so have always wanted things to transition as smoothly as possible between releases.
