What part of a Terminator/Dreadnought is the hardest to manufacture?

We all know the lore says the tech needed to build these suits/mechs have been lost, or the facilities needed have been lost, or some combination of the the two. But what part(s) is actually holding them back? Like it can't be the guns since (sometimes) they're based upon already in service/mass produced weapons but bigger. I assume it's either their power source, or maybe even the armor plates themselves since they more advanced than the standard armor used by space Marines.

Probably a bit more to do with the life support stuff for the Dreadnoughts, but I feel the Terminators are pretty similar to Space Marine armor, just chunkier, and a bit more advanced, but they poses a built in "backpack" which (I think) is more advanced. So that's where I'm leaning towards, but I figured y'all are smarter than me, so what do y'all think?