TL;DR: Telegram should only be used by those who have a very low need for security and only exchange unimportant messages.
My evaluation and opinion:
Surely it is better to use Telegram than WhatsApp but not much. First you need a mobile phone number to register. In most countries you won’t get an anonymous mobile phone number. But even an anonymous mobile phone number is useless if the police arrest you together with your mobile phone. Unfortunately the protesters in Hong Kong did not consider this. Despite this news, protest movements such as Extinction Rebellion and Fridays for Future unfortunately still rely on Telegram. This is dangerous because many states associate environmentalists with terrorists and have them under surveillance by their secret service. Another vulnerability is that a secure messenger is advertised, but group chats remain unencrypted. The central server structure makes it very easy to monitor the communication. It was sympathetic that they rebelled against the Russian government but finally in vain. Because of the central structure Telegram could finally be blocked by the government. A no go in the age of #PrivacyByDefault is that end-to-end encryption is only enabled when you select private chat. This should be the only option!
Telegram is only useful as a temporary message channel (use private chat for the encryption), if you change the messenger frequently for security reasons.
The client is free software. You can download the client via f-droid.org
The software is available for all major platforms: iOS, Android, Linux and Windows.
Video chats are possible with encryption.
There is an end-to-end encryption if you select private chat, which is not the default setting. The possibility to exchange any data without encryption.
You need a mobile phone number. Central structure and server software is not (open source) free. Not #PrivacyByDefault.