A bit more factors tbh. User agents have browser, version etc.

iPhone in New York block still have different IP, versions etc

But I hear you, it’s better than cookies and that’s the entire point.

You can also provide a user id, more info here https://news.ycombinator.com/item?id=40434115

This is the same problem that I had with parsing logs. There's no way to know that mom+dad+kid all using the same model device were 3 different users.