Ads on the web have many problems:
Ad selection criteria have been abused for dissemination of propaganda, for targeted malware attacks. Content sites have basically lost the control over what kind of stuff they deliver through the ad-space on their site. Tracking is increasingly a concern for users. Ads bloat sites, slow down page display times and mess with peoples mobile data plans. Ads use power and create heat in devices with a tight power budgets (basically, anything that runs on a battery and has no fans, phones, tablets, laptops). Ads play unwanted video and audio, open layers and windows, popovers and popunders. Ads destroy usability and layout on content sites.