Void Synthesizers have a couple of key flaws. They can't produce goods, which are critical for elevating colonies from [content] to [prosperous]. They're also extremely endgame, and cannot be used to push planets from [prosperous] to [rich]. 
On the other hand, Starbirth can be used to push planets from [prosperous] to [rich].
Similarly, infraspace can also be used to move the necessary extra resources to propel planets from [prosperous] to [rich].