you have spoken about aoe vs single target healing but these are pretty outdated concepts, i think it would be easier to understand if we break it down a bit further
so traditionally in raids there are 4 types of healing
Preventative,
Blanket
Triage
Spot Healing
Preventative healing is encompassed by the use of personals, externals and abilities like shock barrier, power word shield and aura mastery. they aim to lessen the impact of a deadly hit or improve a persons survivability by providing either temporary HP or improving a players effective HP through damage reduction.
Blanket healing is a term that refers to healing a large group of people for a relatively small amount. for most classes this is their bread and butter and where they get most of their HPS it would encompass spells like healing rain, wild growth light of dawn, glimmer of light as well as things like spreading hots. the idea here is that you are providing a steady stream of healing to the largest number of players with the overall goal of keeping the raid healthy. In situations where the raid hp is critical then the emphasis shifts to larger healing cool downs to stabilise the raid such as Holy word Salvation or Tranquillity. (in this scenario as a paladin we would want to generate and spend HP as fast as possible on LOD but because the whole raid is critical we would almost always hit 5 targets with LOD with or without shadow breaker, as the melee are stabilised and your attention shifts to the ranged, you will most likely swap to triage healing to grab the people who have slipped through the cracks rather than continuing to spam LOD)
Triage is healing that specifically prevents death. usually it means breaking rotation and doing what every you can to keep someone alive until they can be stabilised by either your self or another healer. this is where spells like lay on hands, light of the martyr and blessing of protection come into the mix.
Spot healing refers to often single target healing where you are dealing with a mechanic, this is either a situation where you expect someone to take a lot of dmg or a situation where someone cant die or it will cause a wipe. an example might be the bleeds/magic dots on stone legion for the former. or a user dealing with orbs on dark vain as an example for the latter.
in order for a raid team to be effective it needs to be able to provide a mix of these healing styles. if all we had was blanket healing then people who need spot healing would die and visa versa. as it happens paladin excels is spot triage and preventative healing while not being bad at blanket healing which is why is it such a strong pick every tier (that and its dmg)
the reason shock barrier is so good is because it adds preventative healing to holy shock. this means that in 1 spell we get the spot healing from the holy shock cast, blanket healing from glimmer of light and the preventative healing from shock barrier. while also feeding into LOD
now if we look at shadow breaker, it dose 2 things, 1 it increases the range of LOD allowing you to more easily hit 5 people. and 2 it increases your mastery effectiveness both of which feed into blanket healing
one very important thing to remember is that LOD in a raid will almost always hit 5 targets epically in mythic where you have 20 players, in smaller raids this might be a bit more difficult but you should still have no problems
As a paladin we move to maximise our mastery effectiveness anyway. so in essence the 2 things that shadow breaker improves are 2 things which should already minor issues and can be fixed with proper positioning making the legendary a lot less effective
I’ve linked a log of mine below, (unfortunately Warcraft logs is timing out at the moment hopefully it is working when you can look at it)
https://wowanalyzer.com/report/jHtwmLATXN349VRq/6-Mythic+Shriekwing+-+Kill+(5:50)/Alexiefish
If you navigate to the statistics tab, you should see mastery effectiveness and the average number of targets hit with LOD. those are the two metrics that Shadow Breaker is looking to improve. This pull was done with shock barrier equipped and with Shadow breaker we could expect them to be slightly higher but they are already quite high, meaning the actual value that shadow breaker is adding is minor at best.
TL;DR Shadow breaker healing has next to no meaningful value because it works to improve things that are already very strong in the melee build.
if i may offer an insight, i think that shadow breaker has been designed to work with the holy light build. and i can see its value when the ranged is spread far from the melee. but with the glimmer build it just doesn’t do anything significant enough to justify taking it. it will not be the reason someone didn’t die.
i know i went on a massive tangent there but i do hope this information helps you to understand why we avoid shadow breaker in favour of shock barrier. i could have gone into a lot more detail discussing glimmer and divine toll, judgement of the light and all sorts but i think this post is long enough 
if you made it this far thanks for readying and have a lovely day