I've worked in ad tech for years. For the reach you get Google is too big. It also is piss poor at data values returned. Companies like appnexus can give you exact impression (views), clicks and conversion figures that rarely change, and filter out fraud automatically. Google? You have to wait 3 days for their data to stabilise, and even then they often changed results a week later with statistical significance.

their estimates on volume are equally bad. There is so little confidence in the results where I've worked that many came up with their own algos to managed the algo generated data from Google. I can only guess it is even worse for seo and sem peeps, as dozens of companies exist just to facilitate reporting and support on this.