The set itself actually has the same base resistance as the Soldier's Armour set, at 4 defense
per item, but it's the bonus effects that make it
stand out: Guardian Resist is a base effect applied to any item you're
wearing, giving you a resitance to damage from all Guardians, whilst
wearing a full set will earn you the Ancient Proficiency set bonus that also increases your damage with Ancient Weapons and Guardian Weapons.