From what I checked out on Youtube I would say Akuma seemed to have the power advantage.
Based on that cutscene alone, Akuma absolutely had the advantage. He had the stronger blows, as well as
tagging Heihachi more than he was hit.
Then the fight turned into two players playing them, and showed things like Akuma not blocking a single
one of his blows, to there being PS4 command prompts on screen. At this point, the fight stops being
serious and is just showcasing gameplay.
That being said; does Heihachi lack a soul? Because he got hit by that one move of Akuma's that tears your soul to shreds and managed to survive it.
Gill survived because he's got this weird power to resurrect himself, and Gouken survived because
he used a specific technique to do so.
Heihachi does have a soul. But keep in mind that Akuma's Raging Demon was done during gameplay, not its normal in-canon power. So instead
of dying, it just "does damage" to Heihachi.
Also exactly how are street fighter and Tekken connected on a canon level? I mean first Capcom x Tekken and now this?
They aren't. These are what-if games. The Capcom vs. series, the crossover series in their various RPGs, etc. None of them are canon
in accordance to the official series.