IMO, the only advantage that Discourse Hub has right now is that it’s easier to (learn how to) install. Discourse Hub is an app, and everybody knows how to install an app. In Hub, you’d then have to paste in the forum URL, but there’s UI holding your hand through the process. Whereas the iOS A2HS process is… non discoverable. (Hence this thread!)
“Multiple sites in one” is actually a drawback for my tastes. (But, if you’re interested in it, it is now possible to rewrite Discourse Hub as a PWA! 
)
“Push notifications for all our hosted customers” applies equally to Web Push, right? And even non-hosted customers get instant push with iOS 16.4 Web Push, whereas when I’d add non-hosted forums to iOS Discourse Hub, it would have to poll my non-hosted forums periodically, introducing delays.
“No need to log in a second time” … I just use “Sign in with Apple” and I’d say the experience is equally good for A2HS Web Push as it was in the Discourse Hub. I click Login, Apple scans my face, and I’m in.
“Share link in footer” I’m sure I’ve never used that! 