It’s definitely different in different countries. In the US and Canada, it’s the C-word, one of the few insults that Cannot Be Said. In the UK, Ireland, Australia and New Zealand, as you point out, it’s way less offensive. Not something you’d call your grandmother, but something you’d definitely call a friend.

Partly, I think, it’s because it’s less gendered. No one in the US would ever call a man a cunt. But you can, and do, in the UK. I’ve never known it to be specifically an insult for women there the way it is in North America.