For all that horses are big and strong, they have very delicate digestive systems and respiratory systems. The blood flow to the legs would also be compromised since they would be up in the air and gravity would be at work, pulling the blood back into the core.
While rumor has it that the lungs will fill up with blood, they don't. They do, however, fill with fluid after a couple hours. It's called edema. Edema is swelling resulting from a buildup of fluid in tissue. In the case of pulmonary edema, it's the accumulation of fluid in the lungs.
With a draft horse, because of the weight, I would consider putting on two straps and hauling the horse backward far enough that he can get up on his own. If he's been down a long time, that may not work, because he'll start to numb out, but if he's got use of his limbs he's better off getting up on his own. This will work for any horse that is mobile and should be considered a 3rd method for dealing with cast horses.
When not to use it? If you haul a horse backward with the idea that he will right himself, but he is numb and can't get up on his own, you won't be able to push him back against the wall to give yourself room to roll him. Then you have another whole operation. If you don't have enough room and you roll him, he may become cast again, only with his feet pointing toward the opening! Then you'll be faced with getting him pulled out the doorway – a forward extrication. It WILL work if he is in a pipe panel corral and has caught his legs in the pipes. You would then have room to maneuver him, and you could always take down the pipe panels to allow for extra room.
When not to try:
1. any time it would violate the primary rule (Your safety is number one!)
2. if he's injured and you will further injure him by rolling him.