Why don't the iron hands protect their augmetics?

Except from the fact that they wouldn't look like iron hands, I suppose it has a cost to replace the limbs of a hole chapter, and even if they don't care about cost, an arm in less is still less practical in combat and I believe cybernetic augmentations. Are their augmetics made of the same material as their armor ? Or is there a more practical reason to expose them like that ?

Except from the fact that they wouldn't look like iron hands, I suppose it has a cost to replace the limbs of a hole chapter, and even if they don't care about cost, an arm in less is still less practical in combat and I believe cybernetic augmentations. Are their augmetics made of the same material as their armor ? Or is there a more practical reason to expose them like that ?