Telegram bills itself as secure, which comes with a rather large burden of proof if anyone is to take you seriously.

As the saying goes: "don't roll your own crypto, unless you're a professional cryptographer, and even then…". Telegram are not cryptographers but decided to roll their own anyway and have come in for not a little flack as a result.

Search for "telegram app crypto criticism" and you'll find plenty of examples of this.

Is Telegram secure? I'm not qualified to answer that, but I understand enough to be sceptical.

I liked Telegram but all my friends were on WhatsApp, so stuck with WhatsApp for that. I agree on your point that Telegram does not send nasty letters like this.