You use punbb at your own risk.
I recognise that is a concern. The same goes for FluxBB which I have also considered. As you say, Flarum remains in beta and looks as if it could be more difficult to install because of the additional dependencies.
Pun has an excellent responsive theme by default, which I give high importance, but Flux appears to be supported by more users, although that does seem to be dropping away currently. Of PunBB and FuxBbB which would you choose to adopt at this point?
From the old forum engines I can recommend phpbb, there are both developers and communities.
Of the bigger players, I like SMF because it has a similar facility to Pun and Flux for creating static pages that allows you to create a main web site with integrated forum, rather than a forum alone which, from what I've seen is all phpBB does.