One of the biggest challenges facing streaming advertisers today is how fragmented streaming audiences are. Which may be surprising.

With Amazon’s Prime Video adding an ad-supported tier earlier this year and automatically opting in existing subscribers, there would seem to be an abundance of large-scale ad-supported streaming services. And there are. But even at scale, there’s fragmentation.

Watch UM Chief Investment Officer Marcy Greenberger’s interview on Digiday.