There’s a distinction here though - ignoring collision doesn’t mean it doesn’t have physical properties, such as mass, COM, inertia tensor, etc, and it can still affect the simulation via constraints. Doesn’t negate your overall point at all, just pointing out that there are some requirements in order for something to be simulatable.
But yeah, I agree it would be ideal if it were possible to build aggregates like this at the component level with the root being nothing more than a UE4-side reflection of the state of the overall body. I’ve hit the same issue and have resorted to the same approach of a dummy body with no collision and minimal mass. I’m not yet 100% sure this even works without affecting the properties of the resulting rigid body in some undesired way.
I am fairly sure this is just a limitation imposed by the way UE4 maps between its primitive components and PhysX, nothing to do with PhysX itself.