Dps loss. A pretty chunky one at that. And you lose more than 180 potency at 3 dot ticks remaining as it costs you Glare casts over a fight.
Even using every single AF for ED puts you above WHM in terms of dps neutral healing.
Which is one of the reasons why AST/ SCH is simply stronger and has the benefit of requiring less special adjustments for a healer
Which is completely besides the point because their healing is and remains dps neutral, no matter how bad that SAM over there screws up his rotation when you had just given him a card.
Dps ST & aoe neutral healing, which is something like using Aszlum during a natural Dia refresh. Not aoe damage. Don't jump to conclusions.
It's not about "really wanting something", it's about how much you can do until you start suffering a dps loss. And that's where WHM falls behind, like it or not.
Of the 1900 cure potency, only 1200 is dps neutral if used during a Dia refresh. 1900 is already a dps loss.
This is not about how much you can do if you yolo your dps - it's about that WHM falls short when it comes to dps neutral healing, which is a direct result of the poor amount of free weaving slots and the lack of mobility compared to the other two healers.
And how much dps neutral healing a healer offers is escpecially important as long as you don't outgear content like crazy - going into fights with the party near/ full BiS doesn't make WHMs weaknesses disappear, they're just less obvious.
A heal powerhouse is not definied by "if everything aligns perfectly, you stack everything you get, lock yourself out of dps completely, have 10s of glory nobody will ever need and then be completely drained of resources" in FFXIV.
The true healing powerhouse is AST because they have by far the highest dps neutral healing and very strong GCD healing via Synastry and Neutral, something you exclusively use for GCDs heals and is thus always ready in case you are forced to stop dpsing. Unlike e.g. Asylum which gets regulary used throughout a fight. Followed by SCH, which has immense oGCD healing potency, although a lot of it is tied to a dps loss. So 2nd place for the fairy friends.
This discussions are best done in reality, not on paper. It's about as useful as saying that WHM slaps a dummy harder than SCH or AST.
It's not.
WHM gets out-dpsed at all percentiles by both other healers on all bosses.
But the metric you really want to look at is "Healer Combined Damage" because healers don't function in a vacuum. They have a pool of resources that get used by two different players. If WHM was ahead at e9s (which it's not), AST/ SCH is still the better comp in terms of rDPS which is what really counts at the end of the day.
And the differences were far more extreme early tier when not everyone and their cat farmed Promise in near/ full BiS. Right now you have the luxury of overgeared people that have done the content for months. That sugarcoats the weaknesses it has but content wasn't balanced around outgearing it.
This is not about insulting your chosen healer class, it's about bringing weaknesses to the attention of the players and devs that the other two healers don't have or only have to a much lesser extend.
The statistics say that a lot of people play WHM. So everything is fine, right? And if you encounter a problem the other two don't encounter or only to a much lesser extend, you can just do this suboptimal thing the others don't have to do in order to achieve the same, so no problems at all, right? After all you have options.
"WHM is fine, l2p" isn't going to help here.
WHM is crazy good in level 80 dungeons with a half-decent tank because you can dump lilies between pulls and just fire away. It still needs to clip oGCDs while aoeing if Misery is not available and is very immobile during aoeing. Does it make a big difference? No, because the content is faceroll.
But it means that WHM still has these issues even in the easiest and perhaps most tailored-to-WHM content that exists in the entire game.
WHM is not garbage but is has issues that absolutely need to get adressed. Sugarcoating them won't help anyone.
But to stay on topic:
Physick heals with a higher potency but Adlo has a shield attached to it. So even if the HP doesn't go up as far, it barely goes down at the same time which makes Adlo spam overall better.
It's generally best to use WD early into the pull when damage on tank his highest, slap the ground and Lustrate until the tank starts dropping, use Fey Illumination if available and then Adlo as much as neccessary while squeezing more ground slaps in until everything is dead.
Front loading the pull with regens and dps makes it much easier to handle because you signifcantly extend the time until you need to GCD heal at lower levels. By the time you're forced to Adlo, some mobs should already be dead.
None of the skills have crazy high healing on it's own, it's about cycling through everything and combining it for maximum benefit.
Better late than never:
I believe the main issue is that WHM got stripped of several things with each expansion. You once had more dots for more mobility, Largesse was a huge heal buff that is now part of Temperance, BRD had refresh. I haven't played any old ex recently, so this is all theory.
The basic toolkit of both AST and WHM is the same, but AST has less MP issues from Draw and lower MP costs, more mobility and two GCDs to complement it, ED and Synastry. WHM could have higher rDPS as the diffence between their fillers is bigger at lower levels and AST can't use Div during opener for the lack of Sleeve Draw. But AST has an easier time keeping dps up and doesn't suffer from the same MP issues as WHM does pre-HW.
So I think AST should be better here aswell but I'd have to try it myself.