The Popunder ads are an interesting format because they work differently from traditional banners or ads placed directly inside content. Instead of interrupting the page a person is reading, the advertising page usually opens in another browser tab or window. That can make the format less visually disruptive, although the user experience still depends heavily on frequency and targeting. If the same visitor receives too many popunders, the campaign can quickly become annoying. From ads marketing perspective, I think it makes sense to pay attention not only to traffic volume but also to the quality of that traffic, landing-page experience, conversions, and how often the same users are exposed to the campaign.