These: https://www.androidauthority.com/beeper-app-opinion-3345142/

First, the elephant in the room needs to be addressed: security. In Beeper’s start-up guide, the first thing you see is a huge alert box: “Beeper may be less secure than using encrypted chat apps by themselves.” Fundamentally, there’s no way to fix this. To use any of the chat apps, you need to link Beeper to that service using your credentials, which is inherently more insecure than logging into the app directly. Beeper is quick to defend itself by pointing out its robust privacy policy, its ethical business practices with a user-centered focus, and its use of end-to-end encryption (E2EE). However, that doesn’t protect your credentials from hackers that could gain access to Beeper and send your grandma a message through WhatsApp pretending to be you and asking to wire $1,000 to an account in China.

More in depth: https://www.reddit.com/r/beeper/comments/13hhx9e/transient_key_retention_a_suggestion_to_solve/?rdt=61709