It seems like whenever telegram is brought up here, there's a lot of speculation about it not being trustworthy but no concrete evidence.
The client is open source. While not end to end encrypted by default, it shares that property with most messengers. E2E limits search-ability and other features so it's a tradeoff.
I have to wonder if part of why this is mostly brought up is due to the origin of the app being a group with a Russian nationality. That's pretty sad.
The app is damn good. The best messenger app I've used - much better than signal. I'm so disappointed by this attitude.