No Escape From Pain.

The dragon heart made him grow blue scales and his skin became hardened like rock. Giant toothy maws, skeletal wings, and tails on different parts of his body grew during the assimilation process. There were numerous other weird body parts that were grown and absorbed in different cycles as his existence shifted to assimilate the two divine abilities. 

Other races can use a dragon heart without the need to go through the changes he is going through. But the dragon heart won't be able to grow with them. It will just be an organic accessory that will empower them. They will never be able to convert it into their own organ. This means they will never become a true dragon and the dragon heart will not follow them when they become Origin gods. 

In turn, their form doesn't need to ripple and change during assimilation. The assimilation process takes minutes at most and they don't get fatigued enough to sleep.