Beeper Mini avoids some of those problems because it’s operating in a fundamentally different way. Its developers figured out how to register a phone number with iMessage, send messages directly to Apple’s servers, and have messages sent back to your phone natively inside the app. It was a tricky process that involved deconstructing Apple’s messaging pipeline from start to finish. Beeper’s team had to figure out where to send the messages, what the messages needed to look like, and how to pull them back down from the cloud. The hardest part, Migicovsky said, was cracking what is essentially Apple’s padlock on the whole system: a check to see whether the connected device is a genuine Apple product.
It’s been out for a long time with limited access and nothing yet. Maybe Apple will change their mind toward it when it’s being used by a large group of people finally. Hopefully not.
Beeper's entire premise is based on decrypting your messages on their servers, re-encrypting them and sending them to you, and pinky promising that they're not reading them.
Beeper can be self-hosted if you have a Mac, so you don't have to trust their servers
Sunbird's app (Nothing Chat) was riddled with its own security vulnerabilities that allowed users to read other users messages, which were all stored as unencrypted plaintext, all discovered by the community within 24 hours of launch
Beeper is actually open about how their technology works and what it's limitations are, while Sunbird/Nothing basically lied about their product and never provided any meaningful documentation
It wasn't just that E2EE was a lie, their own server software was full of its own bugs that allowed third party access to user messages, which were stored unencrypted in their database.