The issue I'm seeing is lack of WHM weaknesses.

WHM heals are supposed to be big and powerful but come at a cost of MP and enmity.
If 2x WHMs can spam medica without quickly draining their resources or pulling heavy aggro from tank, then why would you ever need to take a SCH to a endgame raid?

That's where I'm seeing the imbalance. WHM doesn't have enough shortcomings for a SCH to complement.