Beeper seems a lot better then. Based on Matrix, so at least you know there is a foundation of security there.
Obviously they can still make better decisions than these guys - don't write confidential data to your logs! Don't unencrypted HTTP APIs! But unless the gateways to the 3rd party services run locally, you will never have end to end security.
Edit: Beeper has published their bridges as open source, so you could self-host it. That would at least give you full control of the stack. https://github.com/beeper/self-host
EDIT: forgot to add that there are several beeper specific MSC's that other clients don't render, thus if you want the full experience you either use their service or just stick with a normal matrix instance.
And it’s been working fine with Element as client.
You place your trust in 3rd party cloud servers for so many things. Email, as an example, is far more confidential / important, but most people never have it encrypted.
It’s still a fair criticism, but I still see Beeper + iMessage as more privacy-friendly than like FB Messenger (which has way more users).
And yet, that's precisely where I would go if I were law enforcement or a secret service. Tell them they're being used by terrorists/drug kingpins/CSAM peddlers, slap a gag order on 'em, and scoop up everything. And unlike Apple, Whatsapp or Telegram whoever hosts such a service can't even refuse such an order on grounds of technical impossibility because the messages are in plaintext in memory.
Use Signal if you want end-to-end encryption and don't feel bad about using Beeper for people stuck on iMessage.