The reason it is safe for your thoracic spine is because it has ribs that can more evenly displace the weight of your upper body while balancing on the roller. The lumbar by contrast does not have ribs, so rolling on your low back would mean concentrating your entire upper bodyweight on the spinous process of the lumbar vertebrae. If you want to stretch out the lumbar a safe way to do it would be to buy a half moon pillow, position it under you lumbar, and perform bridges on top of it.