These are purely my observations of the discourse around Signal and should not be taken as a universal truth. Only my subjective reality.
I'm not aware of any major vulnerabilities in Matrix (but I'm not following) closely. I'm also not aware of any in Signal, which I know is frequently audited. There is an SGX attack, but it is often blown out of proportion (highly technical attack that requires an unlocked phone to be in the physical hands of the attacker).
Telegram gets a lot of shit because they prop themselves up as a privacy app but aren't. The default is that things are not encrypted. They use a proprietary encryption scheme. They store user data on their servers. These are not the marks of a privacy app.
I explained how imo Signal has burned the goodwill of a some of its early adopters. Meaning not recommending it to my friends and buying my mom an iPhone. Now there's an argument to be made that it's not very smart to dismiss the app, but that's what happened.
I'd be very surprised if they manage to salvage its image at this point
I do like Signal and I think they have done a lot of good. I do think they have a lot of valid criticism against them but also I think a lot of people aren't providing useful criticism (it is a shame that's happening here, on a forum that should be filled with tech experts). People also aren't realistic. A 25 person team working at a non-profit aren't going to have the same development capacity as a 250 person team.
[0] (maybe go to the bottom) https://community.signalusers.org/t/usernames-in-signal/9157