“For advertisers, controlling the appearance and placement of their display advertising is always a key goal. Walled gardens fragment this control into multiple technology systems, causing, in the best-case scenario, more work to maintain a similar level of control. In a worst-case scenario, certain techniques advertisers believe to be important to their advertising simply no longer function reliably.”
Read more on Adexchanger: https://adexchanger.com/data-driven-thinking/tactical-implications-walled-gardens/