I've had problems, but only with recurring events; I wasn't getting reminders for those so I made them one-time events and dragged them to a future date upon notification.
They run on Amazon EC2 with instances that have Asterisk; Digium is one provider of hardware for these situations but I don't know how that's incorporated into their architecture
In fact, my karma would be about 1,000 points higher if I'd simply remember to submit the online versions of some of the articles. They're typically highly regarded on hn.