I have shown recently that poor shuffles have a pronounced effect for a basic strategy player. The less random the shuffle the worse it is for a basic strategy player. We can't really make a blanket statement on whether a hand-shuffled vs machine shuffle game as it's very possible that the hand shuffle be more random than the machine shuffle.

As far as why the floating advantage exists, it's related to the decrease of number of card permutations as you go deeper in the shoe.