I come across this a lot about Telegram and while I do agree, I think there have been no reports so far about hacks in Telegram's service, and it's online since 2013 or so.
"Even worse, security doesn't provide immediate feedback. A dead patient on the operating table tells the doctor that maybe he doesn't understand brain surgery just because he read a book, but an insecure cryptosystem works just fine. It's not until someone takes the time to break it that the engineer might realize that he didn't do as good a job as he thought. Remember: Anyone can design a security system that he himself cannot break. Even the experts regularly get it wrong." -- Bruce Schneier
Source: https://www.schneier.com/crypto-gram/archives/2009/0915.html
[1] - https://en.wikipedia.org/wiki/Viber#Market_share [2] - https://techcrunch.com/2016/04/20/viber-defends-new-end-to-e...
India, Russia, and Brazil isn't the target market for people like Schneier. If you narrow the market to the US, Statista reports that Telegram has twice as many users in the US as Viber.
I'm from one of those countries where Viber is hugely popular (by far more popular than WhatsApp and Telegram), and I hate it with passion. Kind of like Telegram, its end-to-end encryption was also home-made last time I've checked, but at least it's turned on by default.