How to get the same result as a "simple deform of a Join" when applying a "simple deform to a parent group"

Question

I am trying to do something I consider should be simple : I have an object "made of" other objects, and I try to apply a simple deform function (bend over an empty) on it. For that I have 2 ways of doing : first 1 join all my objects. The result looks fine, but the final shape being considered as a complex polygon, the file explodes in size. second, I just decide that the basic shape is the parent of all the shapes and I apply my simple deform to it, after having set the same origin to all the elements. It ends as a total catastrophy, the parts flying in a big mess and loosing the whole coordination stuff. My question is simple: What Am I missing? why is a simple deform not applying the same way when objects are parents with the same origin defined? My spheres are not sticking to the model as they should...