It's better than Whatsapp & Co since it's FOSS. It's worse than Matrix and XMPP since those are federated (and Matrix has e2ee). It's about tied with Signal, Signal is FOSS but hostile to third-party clients, in exchange it has encryption on by default.
Telegram is known to occasionally hand out users' data in extraordinary circumstances, but that's pretty rare overall. It's not the choice for super sensitive communications, but it's decent as a better alternative for SNS. It's very popular in Russia because it has public one-to-many channels and unlike VK and etc it mostly doesn't censor stuff. I'll never use Whatsapp, I have no reason to use Signal because nobody I know uses it and Matrix is better, Telegram is alright in my book.
Also it's possible to buy anonymous telegram accounts, not sure that's possible for Signal.