[Anunayj]

because Telegram doesn't end-to-end encrypt communications by default! If you want that you need to start a "secret" chat, which can only be accessed on that (mobile) device, (and cannot be backed up or recovered)

Forget metadata being protected. Though imo Telegram is great for any communication that does not need to be secret.

People have also expressed concern about flaws in the custom protocol used, though most of the concerns are related to metadata leaks than actual text message communication, which might be ok or not ok depending on your threat.