Aside from eating all your metal econ up while producing units?
I consider that a feature. Metal usage is controlled by pausing! And by hitting the cap and staying there!
As to power... The way the portafab is described, what's to stop it going to an enemy system? If it uses power, Is it blocked from producing there due to lack of power? If not, why not?
For context, in Classic, I'll typically move all ship production to one of my spire worlds. I'll typically station my full cap of mark 2 engineers there for building acceleration. In that context, 800% seems incredibly slow, especially if portafacs are supposed to be expensive and rare structures.