7.0.3 pre-patch suggested viability, but it was obvious to me that they had show-stopping mana issues.
With Legion being out for a while, I can plainly say they are absolutely not viable at all. Not even remotely.
Discipline is a two-step healer. What other healers do in one cast a disc has to do with an atonement-preparation and then a dps-cast.
AoE healing is beyond terribad, even when knowing mechanics, excellent preparation and using cooldowns they cannot perform.
.. and just try healing between pulls. There's nothing to do but mend and radiance desperately as the group runs away to pull another pack, leaving you with no mana.
Three trash pulls, drink. Repeat.
Clarity between pulls has worked wonders, but I'm baffled by a double-application being blown away in just a moment.
I know what I'm doing. I really know what I'm doing. Yes, honestly and for true. Even with the mechanics changes.
I have to abandon this. Maybe in a couple of expansions..
Shadow is also shit, so my priest is toast.