All of the healers are capable in any content. There have been changes to buffs that they had in earlier expansions so the raw potency is less, along with everyone feeling weaker at the start of an expansion due to less gear.

WHM is the most straightforward with raw healing and regens. AST has a lot of toys. I can't really speak for SCH. It's still strong with mitigation and pets, but a lot of the abilities and damage players had to keep track of has been trimmed down.