Ordinary Telegram chats are not secure, secret chats are secure - nobody has ever explained how the latter possibly can be eavesdropped. The problem with Telegram is many people don't know they are to create a secret chat explicitly every time they want a secure conversation.

However, the fact Russia has unbanned Telegram after Durov's claims they have implemented an illegal content busting mechanism seem alarming: we all know some harmless things many good people like (e.g. discussing government's faults, smoking weed or sharing voluntarily-taken nude pictures) are illegal in many countries.