From a DPS standpoint, AV and the vault are not difficult at all.

From a tank and healer perspective " Sickness must be purged!!!"

AV made sense because Malboros are annoying enemies in any FF. And the Vault made sense because you were going up against Ishgards best knights.

In SB the only real enemy IMO is the main protagonist. Every other enemy is someone our allies have trouble against but when they face the warrior of light they are mere flies to us.

I think they didn't make a difficult leveling dungeon due to many new players joining the game, or possibly the jump potion fiasco which surprisingly not many people purchased.