I sincerely wish you luck with allquiet. I just want to make very sure you are aware why people still pay for Pagerduty. To compete, you need to be looking at 99.99% uptime or better (ideally 99.999%, 5 minutes of downtime a year) where 'uptime' is defined as the ability to exercise the entire notification stack. The moment someone's site has an outage and you aren't able to deliver the notification, you lose the customer and everyone they talk to.
I also worry about in-app notifications, but that's well-covered by everyone else's comments.
Pagerduty is vulnerable. Their UI is garbage. But you need to have bullet-proof uptime to take them down. It's a tough challenge and I wish you luck!