The protocol itself is OK (not particularly a fan of using GET to register a new feed, but whatever). However the Google pubsubhubbub service is quite unreliable: feeds often not pinging, with very little visibility (Google's service is something like 99% of WebSub feeds; the next biggest player, Superfeedr, was acquired by Medium a few years ago and has since fallen into disrepair and no longer answers support tickets). WebSub is a good idea but never really gained sufficient mindshare, and the fact that the only significant remaining player in this space is Google hardly inspires confidence in its long-term future.
There's a relatively new service, PodPing[2], which uses some blockchain protocol (Hive) to broadcast feed updates as events. You just connect to a node and listen for events. It works quite well from what I've seen, unfortunately it requires buy-in from the publishers and only a small % of feeds (and more importantly, very few popular feeds) support PodPing.
Ultimately went back to long-polling. I wish there was something better, it's wasteful and difficult to provide up-to-the-minute updates, but it just works.