- Telegram channels and group chats aren't actually encrypted (unlike Signal or Matrix), so government agents can intercept them, or Telegram owners can sell you out etc. (notice that Telegram is centralised and the server is closed source; only the app is open-source).
- Telegram individual chats aren't encrypted by default, unless you specifically select the "secret chat" function.The "secret chat" function is conspicuously hard to find (you have to click your friend's screen name and browse the menu).
- Unlike Matrix, Telegram requires a phone number, which is traceable to your identity. Furthermore, it by default doesn't put a password on your account, which means that anyone who can use that phone number can impersonate you. This includes the cops and State agents. (set a password to your Telegram accounts! use an anonymous SIM for politics!)
I usually recommend Signal because Matrix is infuriatingly confusing, even for tech nerds. tech nerds can use Matrix to avoid centralization and SIM card requirements; but for most people right now Signal is the best compromise IMO.