Now, that the commotion regarding the changes has calmed down a bit, I'd like to look at the impact of the new mechanics on White Mage.
Confession's impact on single target throughput
I've seen some players complain about White Mage's potency on their base line heals compared to Astrologian, so I'd like to give some insight as to why that's not a problem.
Confession is simplified a 400 potency single target heal on a 15 second cooldown per target. Cure's baseline healing got buffed to 450 potency and Cure II got buffed to 700 potency.
Within 15 seconds, you can cast 6 Cures/Cure IIs, which amount to a total of 2700p/4200p respectively. We can expect 1 Confession stack on the first 5 Cures and another 0.2 stacks on the 6th cast.
1 Confession stack is worth 400 potency, while the 2 stacks are worth 500. That means, if we spam Cures/Cure IIs, we get 400 + 0.2 * 100 = 420 bonus healing for free every 15 seconds.
Now let's calculate the impact on a single Cure/Cure II, because we are not actually going to be spamming these spells non-stop: 3120p/4620p over 6 casts, which amounts to 520p/770p respectively.
So our spammable Cure has 520p now, making it stronger than AST's baseline heal.
Mana Management
The next complaint I've seen a lot is that Astrologian and Scholar gain access to our Shroud of Saints, while we do not get anything from them in return. Let's dispel that one as well.
Scholars have less mana sustain on Aetherflow, so they probably need to cross-class the ability as well as Astrologians who also lost their own refresh. That means that all healers have to take the mana talent, which they didn't need to do before. There's no imbalance here.
Now let's look at Assize and Thin Air, which are going to be White Mage's baseline mana tools.
Assize's cooldown got buffed to 60s down from 90s. The cooldown also gets passively reduced further by Cure/Cure II's critical heals and occassionally by the rather underwhelming Lily mechanic. If you are single-target healing, I can see Assize being on like a 45s cooldown compared to before. Even if you're dps'ing it's buffed, because the base cooldown went down.
Then there's Thin Air. The value of this spell is insane. Since it lasts only 12 seconds, we should combine it with Presence of Mind to get another full cast and maybe another Regen for free. In dungeons you can now spam Holy like there's no tomorrow and you still have the other mana tools to continue spamming while it's on cooldown.
Overall it sure looks like White Mages can afford to dps without it being punishing on their mana in Stormblood.
DPS
White Mages are basically getting a strength that Scholar was known for, which is sustained multi-target damage.
They now have two DoTs just like Scholar, but Aero III is actually AoE, which means, they can use Aero III into Holy spam on huge pulls. Scholars only have Shadowflare (60s cooldown instead of 30s) and nerfed potencies on their dot spread.
I'd even expect nerfs on Aero III somewhere down the line, because it's basically a 370p AoE spell that is not reduced beyond the first target.
Healer niches
All of that makes me think that White Mage's are going to be the strongest choice for dungeoneering, bringing strong sustained single-target healing, emergency cooldowns, aoe damage and cc to the table.
Scholars bring their usual damage reduction abilities and a strong tank cooldown with the new Faerie Gauge. In content that requires little to no healing, they still have their strong passive healing and can spend almost all of their GCDs on damage.
Astrologians are getting an emergency healing cooldown that's going to be great for Diurnal sect. Diurnal seems like the best healer for AoE healing, but the class still suffers from their defensive cards being unreliable due to the draw mechanic. Single target they can only compete with heavy sustained damage through the use of the cross-class Divine Seal. Nocturnal sect doesn't seem as good as taking a Scholar still, because they lack the big recovery tools and damage reductions.
Predicted impacts on the meta for Stormblood
I think Whitemages and Astrologians will be the strongest healers in raid content and I don't even think that Astro will go for Nocturnal sect either.
Divine Seal just produces insane value with Regen-like effects and Diurnal Astro will be no exception here. It also doesn't seem to work with shields.
Scholars' niche of bringing lots of damage reduction doesn't seem very useful, when you can just bring more casters, who are all receiving a raid cooldown that reduces magic damage.