Well, it's really easy to build a government bot, which just joins all new chats that are created, to passively collect all the info. And if the group is closed/«open not for everyone», then it's not an open, it's a closed group.

So that's not really making the spying harder for a cop. But if you wish, you still can setup self-deleting messages (eg. deleting the sent in a day), you don't need an encryption for that