Digital In my 16 years as a developer and devops, then the pro's and cons is in 99% of the cases, based on your preferences.
One could argue that many other boards are far better, more powerful and reliable by default, simply because they have a bigger follower base, been around longer and have a much stronger dev team. The use of such a small unknown board like Flarum, that's not actively being developed, in Alpha mode and it's team strongly discourage production use, is based on preference.
Even Flarum is made and based of another board, Discourse, but the developers prefer PHP and wrote their own (Almost identical) board.
In today's society it's next to impossible to "make something new", and in the world of bulletin boards your going to struggle tremendously to be unique. And ultimately, what board is best for you and your visitors, is solely based on your personal preferences.
I personally use / want to use Flarum for my love for PHP and respect anyone that tries to make innovative applications with very reliable and functional PHP instead of hiding behind extremely new, dont-even-know-if-their-fly-by-languages-yet like Go and Node.
HavocK The UI of Flarum is one of the exact reasons that i compare it with Apple, the UI other boards provide is the same mess you see in Android, powerful, but easy to fuck up.