Since the CIA (and probably lots of other organizations, now) know how to compromise your iOS and Android devices, they can intercept data before it even reaches the app. This means they can grab your unencrypted input (microphone, keystrokes) before Signal or WhatsApp can encrypt it.
You know that light that turns on whenever your webcam is recording? That can be turned off, too. Even the director of the FBI — the same official who recently paid hackers a million dollars to unlock a shooter’s iPhone — is encouraging everyone to cover their webcams.