All three of Signal, Telegram and WhatsApp make me a bit off about using them for various reasons. None of them are what you'd call "user's privacy first".
As it is, I'm currently having occasional confidential chats (at someone else's request) on Telegram secret chats, and at least that probably is what it says it is.
I don't think any of these three apps are awful.
They are pretty slick, and useful.
I don't feel too bad actually using them, any more then using say MSN, Yahoo or Freenode.
They just don't meet the advertised bar of respecting individual privacy first. And I find that really misleading in the case of Signal and Telegram in particular, which emphasise the privacy angle, and then without letting you know, sprays everyone you ever interacted with outside Signal with a notification, including professional contacts, customer service agents, people you don't like, spammers, etc.
Anyone who says Signal has good privacy is just wrong. When Signal say they have good privacy, that's false advertising.
Of course, Telegram does the same thing. I have Telegram installed, and I use it, but it wasn't really a choice. I needed to access a forum which is only on Telegram :/ Unfortunately that meant I had no choice but to have people who know me notified that I installed it. Someone messaged me about 30 seconds after I installed it to say hi. I'm not comfortable about this, but as I say, I didn't really have a choice.
Then there's WhatsApp. I was surprised to read an article which recommended that, of the three, WhatsApp is probably the most privacy-respecting of the apps. I have WhatsApp installed after a long period of avoiding it, because of all people recruiters started expecting it. Hmph. I still refuse to grant it access to my contact list, because I'm not handing that over to Facebook. Which means every message is associated with a raw phone number only, and I have to guess who it is from the content :-)
You can tell WhatsApp doesn't reveal so much, by the people who have sent WhatsApp messages without being told that the recipient doesn't have WhatsApp installed and won't see the message. I've known a few people this happened to. One installed WhatsApp and found they'd been sent a message a year earlier, from someone they thought wasn't talking to them. They were talking, but the sender assumed of course the recipient would have WhatsApp.