For an example of how flexible BBPress is, check out the new Beans community forum - https://community.getbeans.io. Thierry built an entirely new front-end for BBPress from scratch and it’s pretty darn slick if you ask me.
Regarding Flarum, I have it installed over at http://community.themebutler.com/ and it’s definitely one the nicest forum systems I’ve used over the years. That said, it’s still in active development, so be prepared to get your hands dirty from time to time. I had a few issues when they switched to Composer for updates and packaging, but nothing a few hours of tinkering couldn’t sort out. Also, like Leland mentioned there is no integration with WordPress yet - which was fine for me, as ThemeButler doesn’t have any front end access yet, but that will of course depend on your own projects requirements whether that’s a deal breaker.
Thierry did mention he’d be will to share the work he’s done on the Beans forum (using BBPress), but the only catch is it needs to be setup as a theme, which would work fine in a multi-site setup, but probably not for most normal sites.
Hope that helps!