Actually it is kind of broken, but unlikely you'll have reached any of the things that aren't working. Artifact traits that are not spell specific damage increases do not currently benefit healing. This includes the artifact 'level' from the number of points you put in it (0.5% damage per stack) and sins of the many. In a raiding situation that's going to be a pretty hefty cut to potential healing, unsure how it was balanced though. PS this is why you might have noticed you're healing for a much lower % of your damage done than you might expect. I was typically getting less than 40% of damage done as healing, even though base atonement without ANY mastery is 45%.
In 5mans I've found the main issue is that once mobs are dead the healing stops, which for most groups means you're entering the next pack without being fully topped, eventually this just outpaces the healing I can do, especially if there are a couple of 'tricky' pulls that I'm already struggling with.
You're also losing out on a fair amount of SWP damage as mobs die before the full duration and you probably aren't being afforded the setup time to apply it to multiple mobs each time you engage a new pack.
I'm sure it's more than doable as disc, but I think the group would need to progress through the dungeon differently than if they had any other healer.
Personally I've found holy to just be far more comfortable, especially in pugs when you really can't predict what's going to happen next.