Keep in mind that, while mecha and metal golems and such have been used in past Castlevania games very frequently....they were animated either by dark magics or possessed by baleful spirits. (Tin Man, Crazy Armor, Living Armor, etc.) They didn't have men in them, piloting them. Having mecha or power armor that is controlled by a human inside it paints a completely different picture than mecha or armor being controlled by ghosts or evil powers. By adding the humanizing element to the mix...it loses some of the foreboding uneasiness created by something supernatural or otherworldly. The theme shifts from gothic adventure to modern sci-fi. (and having a few futuristic weapons used to highlight the point "We're in the future!!!" in Aria of Sorrow is not the same)
That's why this is markedly different from previous games, and probably where most people will derive their ill-content from.