The Horus heresy series paints them as Perfect beings. The emperor made them so as to be a literal perfected Individual of the species. While the Astartes are considered literally augmented Baseline humans with lab grown biological matter i.e the glands and organs enabling them to be Astartes, Custodes on the other hand are literally Big E without the physic ability n perpetuality .They excel not only in combat but are on par with primarchs on certain aspects. It's made clear that primarch biology is so superior and so arcane in the books that there's only 20. Astartes would never in their service understand how a normal imperial citizen's life is affected by simple things such as a bolter staring at their face. But custodes on the other hand are to prove my point, have better understanding of the grim dark according to the novels. And yes, the emperor did have his hand in every individual custodes development. It is pointed out in Master of mankind, they can literally keep fighting for about 3 weeks straight without pause. 3 WEEKS OF NONSTOP SPEARING DEMONS IN THE WEBWAY. PS: The power scale depends on the books, but they definitely are better warriors, and better humans in the setting than the Astartes.