Sure, but it shows global replies, it provides global search, it's not O(n^2), it's not easily DOSed, and it's highly amenable to spam filtering, which are the issues you raised.

It's true that this solution doesn't work for private posts and DMs, but the n in O(n^2) is much smaller there, so I don't think it's as much of an issue for personal data servers to communicate directly in those cases.