Besides Slack and Discord and Teams and whatever the heck Google has these days and iMessage and...
I think you mean it's the only messaging app that purports to have a focus on security where messages are stored in the cloud, which is true, but also sus. There's a reason why none of the others are doing it that way, and Telegram isn't really claiming to have solved a technical hurdle that the E2E apps didn't, it's just claiming that you can trust them more than you can trust the major messaging apps.
Maybe you can and maybe you can't, the point is that you can't know that they're actually a safer choice than any of the other cloud providers.