They are pretty good, in my experience, at *eventually* delivering all updates. The outages take the form of a "pause" in delivery, every so often... maybe once every 5 weeks?

Usually the outages are pretty brief but sometimes it can be up to a few hours. Basically I'm unaware of any provider whose webhooks are as reliable as their primary API. If you're obsessive about maintaining SLAs around timely state, you can't really get around maintaining some sort of fall-back poll.