Facebook is slowly allowing more publishers to insert ads into the middle of video clips running on the social network. But among media and ad executives, doubts persist about whether the 15-second mid-roll “ad breaks” will make enough business sense to persuade media companies to produce longer videos.

Among the concerns is the question of whether ad rates will be high enough and frustration that Facebook insists on selling the ad space itself rather than letting media firms sell their own ad space. Executives also worry that viewers will tune out once the ad break hits—something for which there is evidence.