End-to-end encrypted by default doesn't mean much if you don't trust the third party providing the software that does the encryption. One automatic app update can enable leaking of encrypted communication.
The bigger problems for me would be the appstores because they could implement a mitm. Telegram and Signal have their client sources open so you at least can check and compile them yourself but if I would handle data that was so sensitive that I can't trust any other entity I would just self host matrix or xmpp and chat with myself...