While Signal is good for encrypting messages, you can't be sure Signal.org isn't hiding messages from you or otherwise blocking communication, and for Whatsapp, you can't view message history without keeping a backup file. This seems to solve both problems.
If the encryption is ever cracked now everyone can read everyone's messages though can they not? I know it feels unlikely to happen today, but tomorrow's technological breakthrough could make anything happen in a matter of months. The future is uncertain.
The encryption of the messages themselves is largely shared with every other part of cryptography - ed25519 keys are used almost exclusively over rsa for anything new and AES 256+ protects every single TLS connection. A lot of the internet hinges on the encryption used being resistant against known attacks.
