The worst case is still the same, no? API tracker goes down, GitHub has no redelivery, same deal. More a matter of whose uptime you trust more in this regard.
(That's not to say it's not valuable for this use case)
For API tracker, even if their services go down for a short while, it isn't good for business. Though it's been only few weeks using API tracker, we had zero failed webhook deliveries. They say they've designed their systems with this as a primary goal, of course. What if AWS or GCP goes down. It's a matter of trust and SLAs.
AWS does have a 100% uptime SLA on some of its services: Route53, for example [0]. Not saying that ApiTracker could not be a 100% uptime service (in fact, it looks like that's their explicit goal), just pointing out that AWS / GCP do have services that never "go down" barring global catastrophes.
[0] https://aws.amazon.com/blogs/architecture/a-case-study-in-gl... -- Route 53’s foremost goal is to always meet our promise of a 100% SLA for DNS queries – that all of our customers’ DNS names should resolve all the time.