The flow across the surface of a vertical ATS is VERY fast. It is a very thinn sheet of FAST moving water. Very hard to achieve that in the water. The size of pump required to achieve that flow rate at the algae-water interface would have to be huge!
It is my impression that my algae has very little contact with air. It is encased in the sheets of rapidly flowing water. The bubbles that appear are from photosynthesis not a direct interaction with room air. When a spot may be directly interacting with air, it's at a low flow spot and it doesn't grow well. I think as you go away from the vertical model the necessary flow is difficult to maintain and other methods, like surge, might work better.