WHM was okay during ShB because the devs decided to fix their job mechanic: lilies. But that's all. They even took Aero 3 away.
Clearly, they want WHM to be starter healer that anyone can play no matter how ill-equiped you were and fits into that gross healbot stereotype.
Like TyTaurus said, WHM has no niche. Not even prog since dying is punishing with their horrible MP economy. For a healer that's supposed to GCD heal, it has the most expensive healing spells.
It's like the devs saw the positive response ShB WHM got and ran with it even though all they did was give him a meaningful gimmick that reflects the same thing it's normal kit does–direct heals. I get that WHM may not need super complexity like AST but on AST, you actually have things to do and get rewarded for doing them.
AST's MP recovery makes sense because it's encouraging you to draw cards and then rewarding with even more MP with Astrodyne for playing the cards.
SCH has Aetherflow on a 45s CD and has Energy Drain to drop unused stacks before the next refresh, encouraging you to keep Aetherflow on CD.
SGE gets MP for just using their oGCDs, encouraging you to use them often and not sit on them.
These are all healthy habits a healing job should teach the player. And for a starter, training wheels healer WHM, doesn't encourage that at all.
Thin Air should be used for the expensive healing spells like Cure 3, Medica 2, Raise hence why there's two charges. But often times, I just see new WHM players not touch it at all or use it after death only to realize that it only works on the next spell. On top of that, Assize is the only thing that gives you back MP for using it and you should use Assize on CD as its a 45s CD, holding it too long or strictly for healing actively gimps your MP recovery.
WHM should be the job that instills good healer habits but it does the opposite: over-reliance on GCD healing, spells being so similar that they don't have a microcosm of even having different ways of healing. Hence why new WHM players get overwhelmed playing any other healer.