Well, movie pirates use it so it must be safe! /s
Telegram has a lot of pros, but it can be safe only as long as you and every person in your circle sets it as such. Chats are not encrypted by default, you have to set it in group settings. Your phone number isn't hidden by default, you have to manually set its visibility to "Nobody".
It even asks to let it pull your contacts, Facebook-style. There might be several such gimmicks, but generally they are easy to notice and control.
The biggest advantage(?) Telegram has is that everything is saved server-side instead of your phone. So you don't need to keep having to back up your chats and be scared of losing everything if you lost your phone.