Opposite end in terms of security. Telegram group chats have no E2EE, private messages aren't E2EE by default (you have to initiate it as a "secret" chat), and the encryption itself is home rolled.
They do publish https://telegram.org/blog/tdlib so couldn't a client author just do shared key encryption or something?
Also opposite in terms of feature-richness.
Yup exactly, their home rolled encryption is problematic in and of itself, but the fact that it lacks E2EE means you shouldn’t even trust it in the first place.