Marketers who have been pouring huge sums into digital advertising are wrestling with several recent events that add to a troubling picture: some are finding they can’t be sure how well that money was spent or what they’ve received in return for it.

The lack of transparency that plagues the advertising industry was on stark display this week. Revelations that Facebook Inc. overestimated by up to 80% the average time people spent watching video ads on its platform shocked the media and marketing world.

Meanwhile,...