I assume it's the lack of end-to-end encryption by default on basic features.
Good service btw, but not the best from a privacy point of view.
Besides that there it's also them choosing to roll their own crypto instead of using established cyphers and protocols.
https://telegra.ph/Why-Isnt-Telegram-End-to-End-Encrypted-by...
This is exactly it.
Besides that there it's also them choosing to roll their own crypto instead of using established cyphers and protocols.