For me, a key negative witb ActivityPub is the complexity. The RSS/Atom spec is pretty simple, and it fairly easy to see how it could be extended to allow for similar functionality. What are the disadvantages to an RSS based system?
RSS is poorly specified, and cannot handle any feedback.
Atom is well-specified, and has provisions for feedback. But what user agents / popular libraries / whatever else actually uses the feedback (commenting) features? Which servers that serve Atom feeds support that, too, and how well?
