Forge can also be very bad when playing with KC. Sometimes you have to take 2 additional coppers...
Kc/Forge/Fortress is just insanely good though, it is almost as strong as Kc/Bridge at mega-turning.
Even without that, it is not uncommon for Kc/Forge engines to get idiotically overbuilt once you forge away the dross. With Kc and a card gainer, you can pretty easily hit a 4 province turn Kc-> Kc -> Kc -> Iw (gain 12 value) -> Iw (gain 12 value) -> draw (draw 24 value) -> forge (gain 3 provinces), buy a province with your coin.
Another fun shot with Kc/Forge is piling down by replacing your cards (e.g. forge Colony -> Colony). Being able to deny my opponent a province, forge a province (say from a silver and a engine component 5), and take a copper is a VERY strong play late game.
Now all of these are a bit uncommon, but Forge has nothing on stuff like Madman which extremely rarely in a Kc deck wants to swap 1 card (2, 4, 8 ... depending on how many Madmen you intend to play) for just a few more actions.