At the end of the day, the Telegram team's responsible for building the code, generating apps and pushing them to app stores. I'd say the chances of the app being tampered with are rather low. Conversely, since the code is out in the open, the chances of vulnerabilities/exploits/leaks being discovered and fixed are higher.
Disclaimer - I'm a regular Telegram user, been using the app for over a year.
Disclaimer - I'm a regular Telegram user, been using the app for over a year.