It seems this is highly dependant on lots and lots of factors, where most of them are out of your control. Such as how much the mobile company have strangled their version of the OS (*), and so on. You should probably not rely on them for messages that are important to reach the user.
(*) They sell many more phones by claiming long battery times, which is why they strangle and kill lots of stuff for the apps on their devices. On the other hand, they wouldn't sell any extra phones by saying "we guarantee all push messages will arrive!"