I can vouch on behalf on Boneyard and physical/magic spells. I'm currently working on a few alternate setups to provide gear and skill options for a UHC paladin guide as there hasn't been one yet. I want to include 5-8 specs so not everyone wants the same thing. With that said, I'm currently working on a physical/magic caster since I can't think of any monster in the game that is immune to both without a specific element to the fight. People just overlook these spell types because you can't easily get pierce to enemies for them.
As far as holy paladin is concerned, can solar flare just be made so it doesn't lock out other skills? Or maybe it has a mechanic similar to werewolf's lycanthropy that requires it to be maxed? Another option might be adding 1 second per X base points, kinda like was done with graveyard having more punishers.
Has anyone tested skills like cherub strike from the Zod RW paladin shield? I assume solar flare will also grant the sparkle effect like it does retaliate since both retaliate and cherub strike have the "activates when struck" mod. I can't think of all the skills off the top of my head, but it seems unholy just outshines holy caster simply because of the skills in the unholy tree. One point in symphony of destruction late game puts it at about 1.2 seconds per cast with high damage for an off element. Storm lord is just... Storm lord (and yes I know one of those paladin skills, dragon form maybe?, gives attributes but it just doesn't stack up like it does against stormlord), and the tainted blood buff with Ed, regen, and poison resist.. I just dunno what in the holy tree will make a caster as Tanky as an unholy would be. Again, this is biased points of view because I've never played a holy caster and only holy Melee til about lvl 40 this season, so take it with a grain of salt if there are skills and methods to balance the trees that I haven't explored or heard about yet.
Edit: some umo's need merfed a bit. Crystal of tears should probably be closer to +2-5 flat dmg instead of +10-20 since people are getting zomg crafts with +180-360dmg