Unencrypted by default, dubious e2e encryption, proprietary backend, based in Russia and later Saudi Arabia (if I'm not mistaken), which aren't really known for their good stance on privacy.
It's featureful, so it's fine if you don't really care for the privacy of the conversations you're having in there, otherwise you have way better applications/protocols out there

I'm pretty sure it's used exclusively by flat earthers and antivaxxers

Everytime one of my phone contacts signs up to telegram, they are either tech guys/girls or conspiracy theorists.