[markosaric]

downsides are basically that in order to gain the benefits for the user privacy and compliance with regulations, you lose a bit of accuracy depending on the situation. we cannot see whether the same person returns to a site on a different day so count them as a new unique visitor.

i assume that using the referrer header to count uniques has even more downsides as i imagine the number of unique visitors with that method would be much higher than it actually is.