The new "Msgs" look interesting, but I am really going to need an RSS feed to plug into IFTTT for notifications or I am going to miss a lot of them. Mentions are plugged in, so those aren't as much of a problem for me.
I have reply / like / mention RSS feeds all set-up to notify me on my phone through IFTTT. The new messages feature can't do that as it stands. Public RSS feeds might indeed be a bit of a problem for messages... HTTP Authentication on RSS is a pretty well accepted thing, but I believe support is a bit iffy No clue if there is any real solution.
I do feel like there's space for both. Sometimes it is useful to have a conversation with someone where others can weigh in, which makes mentions useful. Other times you just want to restrict it to one-on-one conversation.