I do not have any experience with atemoya on glabra rootstock. The two illama trees I have purchased from Excalibur that were grafted on glabra never survived long. They sucker like crazy. One just died randomly and the other broke at the graft in a storm. Both were 3+ years in the ground before they died, which is a lot of time to waste. I believe I remember reading or hearing at a lecture that some individual glabra trees produce better rootstocks than others for grafting sugar apple, atemoya and illama.
I have a dream atemoya on sugar apple that grows in damp soil, in the run-off path of my daily overhead watered potted plants. It is doing fine. I have it planted on a 4-6 inch tall mound because the area only has 2 inches of topsoil.