It's a huge mistake to frame this in terms of safety rather than belonging. Any criticism of someone who feels out of place will cut them to the quick, no matter how much you pad your language.

If your standing within a group is assured, you can call each other dumb assholes all day and it won't matter. This is the classic phenomenon of male friendship. What works in a group isn't an absence of criticism, it's a sensation of security of your belonging in that group.

How do you make people feel secure? The only real way it occurs is if everyone actually has respect for one another's contributions and work. This can't be faked, unfortunately, and respect has to be earned, and everyone's respect is earned slightly differently. Some peoples' respect algorithms are more compatible than others.

An interesting study might be to try organizing people by what they value most in their co-workers.