One of the complexity of the polling approach on a consumer side is having a long running poller. This is trivial to do in Java apps - start a polling thread - but not so straight forward in case of PHP apps, for example. In that case you'd have to setup a cron job or a separate polling script under some sort of process supervisor like systemd to poll periodically/continuously.

I wonder if the two approaches could be combined to simplify things for consumer apps at the cost of slightly more complexity on the producer side? Instead of POSTing the actual event data to webhook, the producer just uses consumer's webhook to "poke" it - to tell the consumer app "hey, you have new events waiting for you". On receiving the poke the consumer endpoint handler/PHP script can just turn around and do a GET to "/event" with anything > last downloaded event id query. That way you don't have to support long polling on the producer's servers and it's not a big problem if consumer misses couple of webhook "pokes". The next time it does receive a webhook "poke" successfully, it will download all the events and be all caught up. If real time notifications are not strictly required then producer side can even run the webhook dispatching code on a scheduled basis to coalesce multiple events in a single "poke" to a consumer to be more efficient, if desired.