Hi, Thanks for writing in.  We actually have a video on why Telegram is bad for privacy, with the main reasons being:
1) metadata leakage
2) censorship
3) not owning or controlling accounts
4) telephone tied to ID with strict VoIP restrictions
5) banning Tor
6) it’s not in F-droid, google play
7) and even malicious handing over of data to governments:
Learn more:
https://video.simplifiedprivacy.com/why-telegram-sucks/