If you're specifically targeted by 3-letter level organisations, it gets very difficult to protect yourself, sure. "If your adversary is the Mossad, YOU’RE GONNA DIE AND THERE’S NOTHING THAT YOU CAN DO ABOUT IT. The Mossad is not intimidated by the fact that you employ https://", as James Mickens put it in his hilarious column [1].

However, against mass surveillance, Signal will protect you, I'd think, just by making it much more complicated to intercept than WeChat or Skype, which have backdoors built right in.

[1] https://www.usenix.org/system/files/1401_08-12_mickens.pdf