Many people who focus on information security, including myself, have
long considered
Telegram suspicious and untrustworthy.
Now, based on findings
published by the investigative journalism outlet ISt
Telegram literally stores all your messages, metadata, etc. in plain text on their servers. This means that it provides considerably worse security than even proprietary messengers, such as WhatsApp and Facebook Messenger. Telegram has an option for encrypted chats, but it's not available for groups, lacks support for voice and video calls, and Telegram deliberately goes out of their way to make the experience of using encrypted chats as painful as possible.
You're even better off using WhatsApp, but if you actually want a good messenger, switch to Signal. It's free and open source (both the clients and the backend server), developed by a nonprofit organization, and it's basically the gold standard for encrypted communications.
The two decisions Telegram made (choice of infrastructure provider who happens to cooperate with the Russian FSB, and attaching a cleartext device identifier to encrypted messages) taken together reinforce surveillance capability of the FSB considerably more strongly than either of these decisions would have on its own.