I feel like I’ve repeatedly seen on HN that they’re not a good choice for secure messaging (though I don’t remember the specifics around it).
Signal and Matrix are the two options I’ve settled on.
[Edit]: Looks like the main issues with Telegram are that it doesn't use end to end encryption by default and that they rolled their own encryption protocol that's likely not secure. They also used to leak a ton of metadata, but from searching around it looks like they may have made improvements. Either way seems like something to avoid when there are obviously better alternatives.