Yea i have to agree... with the rakata i get an almost streight up 9% additional damage reduction compared to 5-6% shield/absorb from the adrenal. This is with a mix of Artifact and black market gear.
I think the issue is that absorb and shield hit their diminishing returns very quickly in the 650-700 range. I don't even have the new absorb augments in either which will be another 300-400 absorb rating on top of that which means even lower % due to the diminishing returns.
I would have to double check this but i assume armor rating has diminishing returns on the damage reduction. perhaps since shadows have such low armor rating they benefit most from the old absorb adrenal since diminishing returns isnt kicking in much at all for them.
As far as I know, armor has an almost constant contribution to DR. The biggest benefit with the armor adrenal is that the boost is then multiplied by your stance modifier. This gives a nice boost for Guardians and Vanguards and an amazing boost for Shadows.