I tend to fall into this belief about WHM right now. The thing about WHM is that it can and will snowball the other team if it's left alone/not focused as soon as possible. You put pressure on WHM and it becomes a struggle because their main "heal" has a cast time and are required to use their gap-closer ability/nuke to get an instant heal. Honestly, AST feels way better in the survivability department than WHM. It's charged heal is instant; being able to be used on the move, and while it's not as strong as WHM's, it gives regen and should you use a doublecast charge on it- an extra heal with a shield. People underestimate AST; it's entirely mobile, has potentially 2 big AoE nukes that can be used back-to-back, alongside Macrocosmos (with the benefit of more potential healing) in addition to 3 AoE buffs that all have a tangible benefit (outside of Ewer if your team is already maxed on MP). It's LB puts a damage buff on your whole immediate team and damage debuff of nearby enemies and having a Balance card active hurts even more.
I haven't played much of SGE (which feels very composition and team dependent; likely more effective with coordinated/alert teammates), or any of SCH, but WHM and AST seem to be the biggest contenders for "best" healer. A free-casting WHM that's allowed breathing room and allowed to observe the fight on their own terms will wreck the enemy teams, and their LB is a potentially huge gamechanger if they manage to tag multiple enemies. Meanwhile, AST can still function largely the same even if under fire (and being a moving target), and they are constantly providing their team with either a damage buff or replenishing their team's MP for self-heals or Limit gauges. Their LB doesn't immediately change the game when used, but can turn-around (or stack their team even more during) skirmishes. ASTs instant offensive abilities can also effectively make a huge AoE burst.