By the same parallels, in your own words, clerics are not buffed, they're nerfed. They lost extra 10% less losses from 50% to 40%. All that change (war heros have plague immunity) does is drive a change in metagame, being that the game will see less undeads in the server. Undeads were only used this age because they were strong and contagious and cheap and efficient. Now they lost the contagious part, which obviously impacts the part where they're "strong", and then you see that orcs got buffed with more gains, then why use undeads after all?
So by the same logic, clerics got nerfed no? Plague immunity is imbued, but at a cost of 10% more troop losses. But there arent really that many undeads around, so which is more useful?
Get real. Sage should never have same magic science buff as a mystic.