You are right, they also severely undercount impressions. I did a test a year or so ago. I placed the 12 hour cookie on a page that gets a consistent number of views per day to see how many impressions were counted. After 24 hours, I put another pop on the page with a 12 hour cookie from a different company. The second company counted 3x the impressions that PUT did. I have actually noticed this is the case for all of SI's programs - severe undercounting.
In a nutshell, I would not recommend them unless you're totally desperate and cannot get into any other network.