The main benefit of rssCloud is quicker notification of updates versus polling every 10 to 60 minutes, as most feed readers do. The drawbacks are that the tool generating the RSS feed needs to include another element (the cloud element), the tool needs to inform a server that an update has occurred (need a server), and there has to be an app that registers with the server to get updates, and to display those updates (need an app that supports rssCloud). Without all of those 3 pieces, rssCloud is not going to provide the value of quicker notification. You could also consult your fellow employee, Joseph Scott, who worked on the WordPress plugin, for more context.

I am not familiar with what is needed to support WebSub, but I assume there is some work involved, just as there would be for XMPP, Jabber and any other protocol that involves more than polling. That "extra work" is the barrier to entry that helps keep people using Twitter and Facebook - the users get that notification "hit" and network effect without any extra work on their part.