The Moodle app showcases the problem a forum app has, surprisingly well. Moodle is more modular than most forum systems in that there are several key types of plugin, and there are ways for plugins to communicate with the app - but even then it's painful going. Even a dedicated forum-platform-X app would struggle to replicate that right now. Also good luck if you want a branded Moodle app for your site, that starts at either you building it yourself or paying Moodle 5000 AUD for the privilege (plus a recurring fee every year that is at least 1000 AUD). And this is still a healthier ecosystem for an app than literally any of the individual forum platforms have got right now.