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They have a 12K shield coupled with 100% lifesteal, which is generally better then what the other tanks get comparatively and actually allows them to go diving into crowds of enemies without getting melted.
DRK has a 8K shield, PLD has a 12K shield + 10% mitigation, and GNB has either nothing, a 18K heal over 12 seconds, or a 20% mitigation buff.
I disagree with reaper being highest dmg if ignored. Rpr dmg values are low. Any person allowed to freecast/free attack is dangerous. Or should be.
A reaper left to free attack isnt much of a threat. Only if your hp is low and you are out of mana is reaper dangerous, but that applies to every job.