It's not so much that they're harder, it's that they're harder merely because of a factor applied against your damage and a multiplier applied to theirs, because of the level discrepancy. It's lazy, and it's cheap. Rather than coming up with a new mechanic (like Weight of Crota in the lamps, or yellow piss in the Templar room), they just tweaked a few numbers. It's not interesting. And even with the improved drop rates, Crota HM still suffers from a loot sin of old: it grants weapons and armor already obtained even if there are other weapons and armor you haven't received. And it compounds the problem by randomizing stat rolls and perks. So "improvement" is a really lose term to use here. In many ways, TDB was two steps forward and two steps back. Hopefully, Bungie has truly learned from launch+TDB faults and start moving towards a better system.