boc Lack of WYSIWYG Editor is the #1 stopper for using of flarum for me on my communities.
I've used Discourse and PHPBB a lot myself. Discourse is slow but convenient, and phpBB is fast but very inconvenient, when it comes time to post an image.
I'm more of a geeky user, so I can wield URLs to show an image from another place where I've uploaded an image. But a very strong majority of "normy" computer users aren't like me. Most of my friends can't manage wielding URLs - they'd sooner turn to Discord, passing over the world of Open Source entirely with no hesitation. Convenience is king in their minds.
I see Flarum as a hands-down replacement of phpBB (and thanks for that, Flarum devs), but it can't touch Discourse in the way of convenience, until it has that image upload, as per the OP.
I've been looking for something like Flarum - as I trust I could run it on a Raspberry Pi 5, whereas Discourse would be too bloated on a Pi 5. But without that convenient image upload in Flarum, alas, Flarum is a "non-starter" for me. I won't even test it on a Pi5 I have, eagerly awaiting such a test.