I was speaking personally.
There is no doubt AdSense lends itself well to certain types of sites. The classic case I recollect is the one example AdSense used themselves a few years ago. Camera sites.
One person could have a real quality site discussing all aspects of cameras and photography. It would achieve a certain level of AdSense performance.
Another site would be devoted to the latest all singing, all dancing cameras. It is so respected, buyers flock there to find the ins and outs of the latest camera offerings. Visitors are so impressed, and are in the buying frame of mind, that it enjoys spectacular AdSense success.
Like all things in life, it's "horses for courses" and "your mileage might vary".
My sites were never buying sites - technical books excepted, and they were only added via Amazon back in 1998 to monetise the site to pay for ever escalating hosting costs. Had I not done that, I could not possibly have been able to continue affording the costs.
I took a three month gamble that Amazon would generate enough income to cover my costs. It did - had it not I was quite prepared to pull the site down. Back in those days there were nowhere near the number of sites around as we have today, certainly not many in my genre.