> If it were secure, it would only notify that there is a message, with no details included.

You're right. This is configurable via settings, but is not the default state.

That said: if I can get friends and family to use Signal instead of iMessage, that gives me the opportunity to disable those notifications and experience more security benefits.

But I agree with your point: most people think that Signal is bulletproof out of the box, and it's clearly not.