The simplepush server iterates through the application installations and pushes a notification to the server provided by deviceToken. But this is user controlled.

If an attacker registers bogus applications with bad deviceTokens, they can generate endless exceptions when those endpoints can't be reached or can slow the server down by purposefully wasting it's time with slow endpoints they control.

Similarly, attackers can provide whatever HTTP end point they want. Using the server as a DDOS and malware vector.