You could also ask about whether they think it's private. And if they say yes, ask them what it means. Does it mean only sender and intended recipients can read the message, or is it fine if the service has someone check the content. Would they agree on the notion "it's OK my nudes I send to my SO are up for grabs for anyone who hacks Telegram's servers", or do they think should Telegram plug this gaping hole.

Also, people tend to state they have nothing to hide, when they feel they have nothing to fight with. But I can't count the number of times I've seen a stranger next to me on a bus cover their chat the second I sit next to them. Me, a complete random person with no interest in their life is a threat to them.