In those 3 unique swords that you mentioned, Dark sword has clear advantage of the 2 swords. It has 20% increase attack in all stages even if the sword is all you have for dark item. While others (lightning and inferno) are full set dependent and can only be fully utilized if all pieces are acquired. That's why dark set is so special. It's multiple set effects is very good specially Dark dungeon.
We have been asking Nanoo long ago for wings of other unique set but unfortunately nothing was given. Inferno set has an advantage on inferno wings cause that tiny life regen thing can save you, like I saw one battle that the opponent is not losing life and just replenishing his life to full bar 😨 omg.