It’s not, unless they’re some sort of cryptography expert with a peer-reviewed white paper pending publication. The Signal protocol (GPLv3) is extremely robust and has almost no capacity for metadata generation, and both the app and server-side code are under the AGPLv3 (technically if they were compromised they could use different, unaudited server-side code, but refer back to “basically no metadata”). Signal has essentially no capacity to be compromised; they can’t even bait and switch users with a pre-compiled app whose source code isn’t the publicly available one and actually has a backdoor because their builds are reproducible and it would be caught immediately.
Maybe they take issue with the crypto bullshit, which is valid but doesn’t compromise messaging security. Maybe they don’t like that they took away SMS, which I completely agree with, but also actually makes it marginally more secure. Either way, I seriously doubt if they had any mathematical insight into Signal being “compromised” that they would be here hanging around on Lemmy right now.
Be that as it may, it’s still an incredibly short sighted decision to use a centralized service that is under 3rd party control for real security sensitive applications.
Yeah, that does bother me. But it’s also a lot easier to build a centralized service like that than to get people on a decentralized one.
If you really want something private and are willing to jump through a few hoops, Simplex exists. But most people aren’t willing to jump through a few hoops, and even Signal (a pretty low bar) is a hard enough sell as it is. And that’s why I use Signal, because it’s my best chance to get people onto something better. In other words, don’t let perfect be the enemy of better.