Not end-to-end encrypted by default, WTF... you have to create "secret chats" for that:
You can't sync secret chats across devices, Signal handles that perfectly by sending E2EE messages across devices:
Desktop does not have secret chats: https://www.reddit.com/r/Telegram/comments/9beku1/telegram_desktop_secret_chat/ This is likey because it does not store chats locally, it just loads from server every time as of 2019: https://www.reddit.com/r/Telegram/comments/baqs63/where_are_chats_stored_on_telegram_desktop/ just like the web version. So it cannot have a private key.
Allows you to register a public username and not have to share phone number with contacts: https://telegram.org/blog/usernames-and-secret-chats-v2.
Self deleting messages added to secret chats in Q1 2021: https://telegram.org/blog/autodelete-inv2
Can delete messages from the device of the person you sent it to, no matter how old.