Industry standard passive impression
An opportunity of an advertisement to be seen, heard or make an influence on a potential consumer. That isn't enough.
Unclear definition that varies across the industry and makes measurement difficult and inconsistent.
Lack of control.
You don't have any control over where you ad shows up.
Prone to Automated traffic, click farms designed to drive up popularity and traffic of pages, and competitors looking to deplete your advertising budget.
Lack of visibility.
No clear idea of the supply chain.