Is Telegram secure?

This seems to depend on who you ask.

According to Telegram, their app is more secure than similar messaging apps like Whatsapp.

They are based on the MTProto Mobile Protocol to make security compatible with high speed delivery, and the app team also works with the Telegram community to improve the security of their protocol and clients. However, one can find issue with Telegram's claims if one were to look a little closer. For one, the end-to-end encryption that is so widely advertised is not enabled by default, except for voice and video calls, and their “secret chats.“

Both Whatsapp and Signal offer end-to-end encryption by default without the need for a specialized chat or for choosing the option.

Another possible security risk for Telegram is that they allow for massive group chats, up to 200,00 people. The bigger the group of people in one chat, the greater the likelihood that people's data will become less secure or that information might be shared accidentally. According to TrustedReviews.com, at the end of the day, “the decision comes down to what you're looking for in a messaging app, both in terms of security and the features each app offers.“