I have always enjoyed healing in a variety of games, the support role of observing the situation and making judgement calls to help lead my allies to victory.
WHM felt like a real sustainable power healer, solo healing raids alliance or otherwise was always in grasp. Playing WHM with an emphasis on Det and Spell speed and having very low Crit with minimal additional Pie made my heals very quick and the HoT effects from Regen and Medica II consistently potent. I had mana when i Needed it could dump it in emergencies and refill it quickly to keep going and outpace even the roughest groups standing in all the damage.
AST felt a bit meh at first, but once the Anemos gear became available and having separate sets for each, having AST Crit heavy rather than Det heavy made all the difference with its performance. The cards were not always the best, yet even without them AST could hold its own healing, the cards were always just icing on the cake for me and too much icing makes me gag so it was a perfect harmony. Cards for AST were not needed to perform the functions as a healer on the same level as WHM and SCH, while AST could blow through their mana quicker and require more mana assistance, it could hold its own with even the worst cards. Good cards could turn a potential wipe, relying on always having good cards is akin to relying on other players to always be there to compensate for your weaknesses and mistakes, a very poor choice.
WHM did really well at dumping on the heals in any situation and keep them flowing, while AST being a bit more tactful and not just not throwing out any random card. Having the versatility to turn a losing battle into a winning one, they just had to be paying attention to the fight as a whole to see and respond properly. While winning wasn't always int he cards, preparing for that rainy day... When your team cant seem to not stack Vuln.. or When they cant seem to do basic math.. or When they like to sit in AoE's to maximize their damage... Reading your group and saving then playing your cards at the right moments on the right players had the potential to make an unexpected victory out of a chocobo shite storm.
While in 4.x other players could mana shift or refresh to help the healer out, that is no longer the case and is pure down to what the healer has as an individual.
WHM still has a similar feel as in 4.x, just not as godly sustainable.
AST feels...not like AST anymore. The AST update really feels more like they stripped AST's identity that was established in HW and SB. I can't help but feel too many players relied on ST to have good cards so complained over its RNG, getting a sleeve draw of all Spire is not very nice, but its the luck of the draw and it wasn't with you in that moment. AST just feels very generic now with there being no real 'bad cards' for any situation, its all flat damage boosting now.
While the writing for ShB is much better than SB and the roll out much smoother comparatively, the amount of reworking to particular classes and systems has only served to push players away from the healing role. DNC and GNB aside, many past healers no longer want to do it, for proof of that just look at how most of the time DF is asking for Healer & rarely asking for Tank. I loved what I could pull off with AST in SB, now its in the trash can with SCH, glad the new titles for 80 are not appealing.
UPDATE AS OF 10/22/2019 BELOW:
Several months into ShB, WHM is the undisputed king of healing. SCH can perform as well as it did in SB, AST has gotten some love to fix their mana issues.
WHM is doing very well, in all level groups, the only real issues are the players playing them. Wanting to spam Holy, which is complete garbage damage and only useful in a practical application of an AoE stun. Opting to dump their mana and let their tanks, blinded dps die and getting themselves killed as a priority for them.
AST suffers heavily from crisis recovery, granted it may be the fact most AST's prefer to spam Gravity over actually keeping healing up, causing their tanks to die frequently.
SCH's tend to struggle more, yet can manage to perform well in the right hands, similarly with problematic players that WHM and AST has, the key difference is SCH is designed to deal damage to increase their healing ability, the other two are not.
Conclusion at this point:
The pool of quality players continues to dilute. I won't tell players how to play, just correct them on mechanical use as they are the undisputed laws of the world.
For Dungeon content...
If I play as a DPS or Tank I'll pref a SCH healer, sine more times than not they will still perform their role and not tunnel vision and ignore their primary role as healer.
If I play as a healer, will only ever be WHM and I will ensure my tanks, and DPS can push their limits and be assured I will support them strongly to do so.
For Raid and Alliance Raid Content...
If I play as a DPS or Tank I'll prefer any healer that actually heals.
If I play a healer thus only as WHM, I can do without the second healer entirely and win via attrition, even if the dps and tanks have trouble not getting destroyed by mechanics none but themselves could save them from. Obvious caveat is all the DPS don't decide to die right before a DPS phase, yet it seems Eden is currently designed to be very easy.
For Savage content...
It honestly isn't worth the effort of finding 7 other players that can get along and function independently as a cohesive group, if they even exist at all. - Contradiction? perhaps. Having 7 other players learn and know the fights, properly responding to mechanics without trying to push everyone into a 1-size-fits-all strategy; with the ability to adapt as a group should a problem occur ah... If I could split myself into 8 independent entities and cover all positions, I would savage raid.
I believe the stripping of all three healer jobs is entirely because of SCH. They have trouble doing anything to that job because of its shared existence with SMN. Anything they do to SCH, always means it'll negatively effect SMN. Hence..in their minds, to even things out, just strip them all down.
As for "adventure in need", I suspect some flipping back on forth between tank and healer in a few weeks. Tanks will naturally be in short supply, but that will also be healers.
Almost all the healers in DF so far have been SCHs(probably SMN mains), I've played with many 2 ASTs. Definitely only 1 WHM so far.