I assume the physics works against reliability. These springs must be stronger and the upward pressure greater on these flexible shells. Surely they deform a little. Flatten out some, to an oval. Don't they set that way after awhile?
I wondered about this also, but I don't think it would be an issue with buckshot, and obviously not with slugs. Buckshot is packed tightly together, and you would have to compress the lead to deform the shell, and I don't think that is going to happen. The high brass, combined with the tightly packed buckshot, and I don't think the shells will compress enough to matter.
I could see this happening if you stored a bunch of low brass target loads with a strong spring, but I don't see this happening with any shells you would actually want to use in a defensive situation. I gave the mag fed AR type shotguns a hard look, but I like the shape and feel of a traditional shotgun when shouldering. Also, I think I would want a recoil pad on those AR 12s, they look like some thumpers.