Heavenly Devils had their own Heart Image Worlds backing them. When they died, they could extract power from their Heart Image Worlds and regenerate. They could not be killed.
They would not die unless their Heart Image World was destroyed. This was similar to a god who ruled a god nation.
If the gods merely stayed within their god nations and sent out incarnations made of god power to do their bidding in the outside world, they would be similar in nature to the Heavenly Devils.
However, the difference was that a Void Underworld Heavenly Devil was an all-rounded talent. Within the Heart Image World, the Heavenly Devil would have to gain insight on and build up the rules of the world.
On the other hand, gods resembled parasites who lived off the planes. They only had to understand the core rules about themselves and carry out their divine professions. The universe's rules would fill in the rest.
This was the greatest difference between them.