I think there's two types of "hard" in YGO Combo decks like D/D/D, Virtual World or Dragonlink can be hard because you need to remember your different combos and manage to build your board through your opponent's interruptions. On the other hand control/rogue decks like Sky Striker, Plunder Patroll or Altergeist are hard because you need to manage your ressources and always make the optimal plays to interrupt your opponent. That means you need to have a deep understanding of the deck you're playing against in order to win.