Difficulty is subjective. Some people breeze through Nightmare mode on the 3D Ys games like it's child plays, others can't even kill normal monsters in Bellistine castle or Clock Tower or Ruined city of Kishgal.

Rare items are items that aren't in surplus, if everyone has Sword of Reckoning that has an added effect of shattering the land, it's no longer rare, it's common. Time commitment being added to rare gear is what keeps it rare without dropping the drop rate to .01%, i.e Ragnarok Online cards.

If you have gear that will drop at 100% after a certain period of time, it's no longer rare, because after 3 kills it gets to 50+%, what's stopping 100s of others from killing 3 mobs and getting said rare gear?