No end-to-end encryption by default, you have to explicitly start a secret chat. That means that instead of it all being encrypted noise, secret chats stand out.
Servers are not open source (last time I checked). Why not? Seriously, why not?
Admittedly, not much of an issue any more, but in the beginning they had horrible security (so did WhatsApp until Facebook threw some competent engineers at the problem)
Don’t use Telegram, let alone pay for it. So many red flags with Telegram.
Feel free to list them - with evidence and not just prejudice…?
There’s end-to-end encryption. It’s fine that you can chose what needs to be private, and what doesn’t need to be.
There could be several reasons as to why the servers are not OS. Why do you need that part to be OS? Seriously, why?
Oh, so your problem with Telegram is, that it had some issues in the past, just like EVERY other app in the beginning? Nice one. :-)
So, let’s summarize.