Although the privacy stuff is still up there on this one, we have not found a lower CPA than retargeting. And not just regular but there are more CPA-based models coming out so there are efficiencies everywhere!
We have run various retargeting campaigns that are incredibly successful - like a-quarter-of-the-target-CPA type of success.
There's no arguing that the method works: to re-message people who are pre-qualified, essentially already interested in the brand. This article speaks more about channel sellers using retargeting, which is always a little harder to measure.
But if there are efficiencies to be had by ecommerce retargeting, those benefits will be seen there as well. Trick is to jump on it as soon as possible to start building up your pool of audience to retarget and it should be an always-on campaign tactic (and a part of any display campaign). And as always, we love anything that touts display: "Search is confined to one site -- the search engine -- but retargeting works across all participating sites on the web."