There is a “backdoor” in Signal nobody cares about, only Google can use it.
This is not primarily about a specific problem. The problem is that you just can’t trust a messenger that is – even partially – non-free software. Signal is, WhatsApp is, Facebook Messenger is, Google Allo is. Deal with it!
You can of course also fix the Signal app. LibreSignal was a fork of Signal that does not use any Google services and thus does not suffer from the problems mentioned, but releases were halted following WhisperSystems request to do so.