Telegram has various issues for the privacy concerned, namely that group chats are never E2EE and 1-to-1 chats are not encrypted by default, among other concerns[1].
Also Telegram is not open-source.
Also Telegram did some shenanigans with cryptocurrency a few years back[2].
The Telegram apps are GPL. Their server is (afaik) closed-source, but having an open source server implementation doesn't mean much for a centralised service without any server-switching support anyway, so no effective practical difference to Signal here in that regard.
Also Telegram is not open-source.
Also Telegram did some shenanigans with cryptocurrency a few years back[2].
Telegram does have better UX than Signal though.
[1] https://en.wikipedia.org/wiki/Telegram_(software)#Security
[2] https://en.wikipedia.org/wiki/Telegram_(software)#Telegram_O...