We use Xenforo, a commercial package. We'd have to rewrite it every time they released an update. That would be quite painful.
I get how this sounds if you haven't used it very recently. There's no way I'd trust Opus 5.0 with that task.
That forum software is around $500/year (if you want your own branding which I assume they do).
To be fair that is more reasonable than I expected, but still... PHP.