I just reached level 88 on Sage today and I really like it. It really plays like what i wanted Scholar to play like since the 5.0 rework severed anything related to Arcanist from it's kit.
Addersgall is a much more flexible and less clunky than Aetherflow. Whenever i need to heal i heal and get 7% mana back. Overcapping on Addersgall is a non issue since i can immediately restore 1 stack when needed and a lot of the time i don't need the heals anyway since Kardia & Eukrasian Diagnosis is enough to keep at least the tank alive. In case of Mana running low i find Lucid Dreaming more than enough. Toxicon also helps with MP pool since its a return on investment from Eukrasian Diagnosis, a similar mechanic to Dark Knights The Blackest Night though instead of 30% mp we're talking about 4% mp. And tbh if you really need the MP this badly just use the abilities. Worrying about overhealing is a little silly considering how much you'll be overhealing with Kardia anyway.
Eukrasia reducing hotbar bloat is a luxury that makes older healers instantly feel dated and it makes the gameplay slightly more involved since reapplying buffs or shield now takes 2 button presses instead of 1. Who knew that 4.x Dark arts would improve healer gameplay.
Overall had absolutely no issues in dungeons healing and in the first ENW trial my CO healer even let me heal almost by myself. Only helping out in some cases.
I also had no issues getting Addersting procs in instances where Critical HP is restored or my tank applied their own shield onto themselves. It appears Differential Diagnosis and tank shields are the last to be exhausted by enemy attacks. I've had several Addersting procs while my tank still had a yellow shield bar in the party list.

Haven't played other healers yet but i can't believe Freecure, Enhanced Benefic & Undraw still exist. Both Freecure and Enhance Benefic just confuse newer players into spamming Cure I & Benefic I (had that happen on me recently in the Trail Roulette (Titania), someone boosted a WHM and was spamming Cure I the whole fight) and Undraw literally does the same as waiting for draw to come off cooldown. Also with Glare III being 310 Potency and Afflatus Misery not being adjusted to 930 potency feels like they didn't update WHM's toolkit properly. This feels especially bad since Sage, similarly to WHM has no damage related raid buff yet does way more damage across the board.