When people say ads are ROI trackable, they are referring to ads that have specific monetizable actions, such as buying a product. There's definitely some advantage there.
However, most big ad spenders engage in "brand" advertising, where the goal is building brand recognition to influence decisions at the future point of purchase decision rather than driving a purchase decision at the point of viewing the ad. I don't see what measurement advantage an online ad brings for this type of advertising, and I see many reasons why it would be inferior to, for example, TV, which can more fully capture a viewer's engagement than a side banner ad.
However, most big ad spenders engage in "brand" advertising, where the goal is building brand recognition to influence decisions at the future point of purchase decision rather than driving a purchase decision at the point of viewing the ad. I don't see what measurement advantage an online ad brings for this type of advertising, and I see many reasons why it would be inferior to, for example, TV, which can more fully capture a viewer's engagement than a side banner ad.