Unwieldy does not slow the Dreadknight or a Dreadnought down. The full text of the special rule in the 7e book is: "A model attacking with this weapon Piles In and fights at Initiative step 1, unless it is a Monstrous Creature or Walker." Not sure why they went through the trouble of changing things to make them use Power Fists instead of DCCW though, it's not like GW has a problem with having multiple weapons with the exact same profile everywhere else (see Cyclone vs Typhoon Missile Launcher, for one).