I will give my feedback on Healer Path Cleric, Healing & Protection domains. I played a 25 healer 5 loremaster a few months ago to level 30.
I will speak mostly of PvE as I did not have a single PvP encounter (cleric of Adama).
To get a general idea of the PvE performance I will begin with saying I was able to solo everything I encountered in the game except for Paush. Baator generals were my hardest fight, where I had to use timestop and mass haste grimoires to tip the scales in my favor. I recently heard about bastille/bastide, I have no experience of this dungeon. Same for RDI and Lowerdark. I was able to easily solo everything else: Maurs, Llolth temple/gythyanki, vampire citadel (with the children).
As a simple rule : anything that doesn't revoke its summon or directly threatens the healer, healer can solo easily, due to infinite healing from 100% refund casts. Even in high burst situations, hasted Heal will make any non-lethal damage irrelevant.
I can't quite remember but I was able to cast about 20-25 times Heal before depletion. And there is also Mass Heal, Circle of Healing, Cure Critical... For PCs there is also Lifeline...
-> From my experience, Healer trivializes PvE content when grouped with other players. I am certain that I could have done Paush with just one allied PC (because they don't get revoked by wof/can't be turned), because I can deliver over 10000 total hit points of healing and over 600 per round.
-> In my experience it was completely ok to trivialize all content, as what I enjoyed in the Healer is having a formidable power of Life.
-> If one were to design a dungeon to beat a group with a Healer, there would have to be Instant Death / Hard Crowd Control effects, because no amount of damage can beat a Healer.
Someone talked about Bless and Pray. I will include Battletide to this.
Bless : Circle 1 has pretty much nothing to offer at higher levels. So one can slot mainly Bless, and add to that with Extend Bless.
Prayer : Equally Circle 3 is pretty poor, can slot a lot of Prayer, complete with Extend Prayer
Battletide : It competes with Heal and Extend Haste, so it is not such a feebie, but if one wants to, they can maintain it as well.
-> It is possible to maintain 40mn+ of uptime on rounds/level spells.
Concerning summons :
Recently summons got nerfed (= reverted).
As a Healer, this is 100% legit. Summons were completely overpowered. DIre Wolf with 2 APR and Cleave at level 3... Broken.
At no point was I ever threatened by anything, aside from the Dragon of Skal which I had to use multiple Gates to beat.
I have restarted another Healer atfer the nerf to summons. From what I see now, summons are no longer overpowered, and have ok power level.
Summons and cantrip spam is the only damage over time source a Healer has, which makes it entirely rely on summon power for its solo clear pace. However a Healer is not supposed to be a damage machine, so this is fully ok on my side.
Something new however : now that summons can guard the PC, Healer's life has become a complete breeze, freely casting anything they want in melee range. Now, Healer can use Battletide when soloing hard content (or playing with a PC who can't guard you), because they won't get blasted in melee range.
As for PvP I have 0 experience with it, I will however say a few things :
Lawful Healer can dip 3 Monk and reach 56AC iirc.
Non lawful can dip 3 Bard and reach 55AC.
(not counting Battletide)
~~ 70 Concentration/DIscipline for both
What needs to be understood is that, if a Healer can cast, then nothing can die in their group.
A Healer could even use Respite during a Hellball cast and save their entire group from a massive AoE wombo combo.
So, balancing of the Healer is quite delicate, because if their ability to survive exceeds a certain threshold, they become immortal.
And by the way, if you don't care about PvP, 25 Healer 5 Loremaster is incredible...
Gifted 19WIS 18INT start, so many skill points, take gift of tongues for +13 languages total.
Conjuration, Abjuration, Transmutation Epic Foci, Divination and Illusion perks with Loremaster...
If I had to choose a build to roleplay Jesus, it would be this one xD