Two major drawbacks!
The first one doesn't need much explanation; Shen He wants this very effect.
The second one, however…
Free Heaven Realm, Ancient Great Demon?
To be honest, it's somewhat dangerous.
Because Shen He does not know exactly what this so-called assimilation entails.
Is it a simple Divine Soul devouring, physical possession?
Or does it fundamentally change a person from consciousness?
If it's the latter, then cultivating this technique might be dangerous, because it might affect Shen He's true self; after all, Zhuang Zhou's Dream of the Butterfly involves a cross over of consciousness.
Of course, this is merely a hypothesis.
A capability like Zhuang Zhou's Dream of the Butterfly, even in the real world, is considered beyond extraordinary, not something ordinary Immortals, Buddhas, Gods, or Demons can comprehend.
For an ancient demon in this Taiyuan Realm to cross such a barrier to affect his true self, seems even more unrealistic.
