Pop up banners are a good media to advertise but it has many disadvantages. Webmasters use pop under ad network to earn revenue but the cause that made because of the pop ads are very big. It mainly affect the flow of visitors to the site and not allow them to read the content or disturb them. So using pop under ad networks is good to earn but if the visitors didn't came to the site how it is possible to generate income. And also it is noticeable thing that the pop under using sites having troubling with loss of alexa rank , seo rank and sometimes even it may trouble the page rank also. Alexa gives rank according to the traffic that recieves site so if a site's traffic down the rank will decrease and in the case of seo rank it help the visitors to track us if it goes down it only causes the search engine visibility it also affects the traffic because of the decrease of the rank. In order to earn income with a site without harming the visitors use banner ads. It is visitors friendly and it help the webmaster to earn. So for earning the best thing is use CPC or CPM banner ads which is far better than the CPI pop under ads.
Mostly the CPI ad networks are providing the pop unders as it is good to track the visitors and it is very easy to show i.e CPI ad networks has a rate of 0.10$( 0.0001$/visitor) to 1.50$(0.0015$/visitor) so advertisers may came to advertise while on the CPC ads the thing is different it need about 0.02$ to 0.10$ per click based on the geo targetted. The rates are different to attract advertisers but for publishers it is like trap that is the pop ads may cause problems to the visitors to came back and also pop on click make impatient in the mind of the visitors who are searching for an important content. So it is better to use quality banner ads rather than pop under ads. All the best.