Think of it this way: WHMs Assize is the answer to SCHs Fey Blessing + Chain Strategem. It deals the damage that SCH allows other players to deal and is an oGCD heal. Of course it's not totally equal in terms of numbers, but it is what it is. SCHs Afflatus Misery is present in having so many oGCDs to heal with between Aetherflow and Fairy, that they don't need to use as many GCDs to heal - Afflatus Misery is only a catch-up ability to offset the missed DPS from needing to use the GCD to heal.
The true culprit is Holy. The other healers don't have AoE mitigation like that anymore. AST lost its stun on Celestial Opposition, and SCH lost it's Shadowflare. If WHM had Fluid Aura upgraded to Flood or something and it applied Slow to the targets it hit... well AST and SCH would really be out in the cold then for dungeon groups. The easiest answer to this is removing stun from Holy, but the best answer to this is giving AST and SCH some sort of AoE-DPS-as-mitigation spell.