For me, Telegram is just one massive facepalm.

https://www.politico.eu/article/irans-telegram-revolution

The main things wrong with Telegram are:

• Proprietary server, running who knows what
  
  
• All users critically depend on one server, run by some dude (also see Signal)
  
  
• Probably bad boutique crypto algorithm (remember the Snowden quote about boutique crypto being the easiest to break)