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!")
Also I find it puzzling that so many people here keeps on recommending WhatsApp over Telegram after all the lies from WhatsApps owner.
Edit: While I have no way to verify this, AFAIK both Telegram and Gmail stores data and keys in ways that makes them hard to access by everyone except for the user.
Telegram in particular say they do this by storing data and keys in different datacenters in different jurisdictions.
Add to this that WhatsApp has had their fair share of issues as well before they started working with Moxie.
It can take years, much like a car has to be crash tested, a new crypto algorithm must go through a certain process to be considered good enough.
That said: I belive them when they say that WhatsApps crypto is stronger.
On the other hand I would expect them to leave a little note somewhere about WhatsApp being a data collection tool for Facebook that also still happens to works as an instant messaging platform.
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.
But to be clear: I have a lot of distrust against governments regarding mass surveillance. But I distrust some governments more than others. And Russia is relatively high on that list for me. I think a healthy dose of distrust would be fitting for Russians citizens too.