You've hit on one big risk in this new way of doing business. If Heroku or AWS or Urban Airship goes down, we all get pissed. This is why these companies emphasize their reliability and customer service.

However, the better question is "would the service be more reliable if my team had written it?" People feel safer when they are in control (people are more scared flying than driving a car, yet driving is much more dangerous.). Chances are Urban Airship is going to offer a more reliable notification system than you would build yourself because this is the one thing they do.