Blog post by crypto professor Matthew Green, discussing what Telegram does (I wasn’t familiar with it) and criticizing its cryptography. He says Telegram by default is not end-to-end encrypted. It does have an end-to-end “secret chat” feature, but it’s a nuisance to activate and only works for two-person chats (not groups) where both people are online when the chat starts.
It still isn’t clear to me why Telegram’s founder was arrested. Green expresses some concern over that but doesn’t give any details that weren’t in the headlines.
Functionality-wise it definitely is, but secure wise no. There’s probably same kind of tracking for profit going on in the background like with WhatsApp and signal. SimpleX is a better alternative privacy and security wise, but it’s still in heavy development