Some of the folks in the OU's Institute of educational technology are experts on accessibility and assistive technologies.
The regularly review parts of our VLE, and since Forum NG is one of the main tools used by students, it has been reviewed several time, and the accessibility problems fixed.
(IET reviews and testing have also fed into accessibility improvements in core Moodle over the years.)
So, if nothing else, ForumNG is very accessible.
The other area where ForumNG has more features than standard forum (in addition to student-visible features that everyone wants like subscribe to thread) is in the moderation features. Some of our courses are very big, and have very high traffic, and we insist on moderating them, sometime by teams of people. So, for example, when a post is 'deleted' by one moderator, it is actually only hidden, and another moderator could review that decision, and undelete the post. You can also delete any post, even if it has been replied to. You can subscribe to a group in a forum, if you have access to multiple groups and don't want to subscribe to everything.
Another point to make is that a lot of forumNG functionality is implemented using the 'ForumNG feature' sub-plugin type. By default there is a lot of functionality, and if you don't need it all, you could remove some sub-plugins. (Some of the sub-plugins are quite specific to how the OU does things. We recognise that.)