While advertising is an attractive source of revenue for many websites, as it is passive income, it is also a very risky model. An ad network may terminate its relationship anytime, without even giving a valid reason. Alternately the ad network may go out of business anytime, and the publisher will have to hunt for alternative sources of monetization.
Even if the ad network is paying on time, there is always a lot of inventory which remains unused, due to a decrease in the demand. For example, if 20 websites are listed on an ad network, and advertising is sold on only 5 websites, it is very expensive for the publisher, who still has to pay the expenses for the remaining 15 websites like domain registration, renewal without earning anything. Hence it is important to consider the cost benefit ratio while developing and designing multiple websites.