People like tptacek have talked here at length about why Telegram is not trustworthy, you can see a history of his comments with a simple search: https://hn.algolia.com/?query=tptacek%20telegram&sort=byPopu.... Moxie Marlinspike has also pointed out a bunch of problems with Telegram, and even if you don't consider him a trustworthy source because he runs a competing service, the technical reasoning behind his opinions is sound.
If you want a personal POV, here are three reasons why Telegram is a bad idea:
1) The large number of unsound technical decisions. See Thomas and Moxie's many comments for details, or the "Security" section on its Wikipedia page.
2) Within days of launching, they had a critical security vulnerability: https://news.ycombinator.com/item?id=6948742. Frankly, this alone should have discredited them forever, especially considering how much boasting they were doing beforehand, but people are stupid.
3) They have a consistent pattern of responding to criticism not with technical defenses, but with ad hominem attacks and conspiracy theories ("You're paid by the US Government!")
As a Russian, I do appreciate the fear that the "russki" brand instills in your soul, but I think you are rightly being downvoted for jumping to conclusions simply based on nationality.
Telegram in my opinion is far better, it's completely cloud based, you can use it from whatever device you want, it has real desktop apps, you can send files, you have bots, channels, large groups, usernames, you name it.
I don't get why using Signal, yes it's free software, also Telegram it is (ok, the server is proprietary but even if you have the source how can you be sure that what they release is what is running on the servers ? If you don't run your own server the source are useless), but I don't see other advantages, so why bother with a third messaging app ? I use WhatsApp for the large user base, and I use Telegram for the advanced features if I need them.
Also, it has a much more praised security and cryptography than Telegram, is always encrypted (Telegram is only encrypted in secret chats) and has a much more secure codebase, with more open development (Telegram sometimes takes weeks to release source code), reproducible builds and a more transparent history.
I do use Telegram (mainly for group chats), but I treat everything posted in it as I would treat a public forum like HN.
Just go XMPP with OMEMO, so no hard smartphone dependency, no electron app monsters. Thankfully XMPP doesn't have a problem with 3rd party and federation.
I just don't want any more applications open.
I'm CA's will be bypassed soon. There's a lot of brain attention on this.
