SCH is definitely easier for players to be bad or mediocre at while still doing at least the minimum required of them. I'll take a bad SCH over a bad WHM any day since the fairy, at least, will do something even if her player is a space cadet.

WHM is easier to pick up, but is punished harder for making mistakes. It's also punished harder for party members' mistakes. Solo healing raids as WHM can be a pain when new people eat unnecessary damage at the same time that the tank requires your complete attention. You start to feel the constraint of the GCD in a way that isn't as taxing on a SCH.

Props to good WHMs in any case. I never consider it weaker than SCH, just very different. And not nearly as easy as it looks when it comes to end-game.