(Not speaking to CF specifically.)

I think we need to nail down what we mean by unique visitors -- we're talking about an estimation which pools from many different sources to calculate as best we can the number of different real human visitors. There is good data in the server logs but if that's your only source of data then your estimation is going to be coarse.

This can be good enough for certain applications, and is plenty for sites with simple access models but you do lose information compared to what you get from client-side tracking.