Rabble- doesn't it impossible to really answer if we don't know what code runs server-side? 

Also, FYI There is the Molly fork of signal which is FOSS including push notifications, and which still connects to the signal servers.  

 If you find yourself forced to use a network that is under the control of, designed by and/or operated by a potential adversary, then you're going to need end to end encryption.

- Phil Zimmermann 

 Yeah. I think the encryption is unlikely to be compromised. But given the centralized identity servers the metadata could easily be compromised.