The cops don't break crypto. They go to the poorest person in the group chat and say, "we're going to arrest you, harass you, assault you, and ruin your life if you don't let us read your chat."
Relevant xkcd:
Security
I really think the most important things to evaluate about Signal if you are not in a group chat of encryption experts are:
- How secure is every device in the group chat?
- How safe from harm is every person in the group chat?
- How much trust is there between every person in the group chat?
