Mistweavers suffer from the same issue that priests do, or at least holy priests, and that is that they are fighting with resto druids for a raid spot, and druids are just too damn good, while also being generally easier healers to play than priest and monk. i guess disc also sort of competes with druid although it's just far more focused on healing up burst damage, and too reliant on outside cooldowns, will have to see how exactly they turn out after 7.2.5 but so far they are kind of a mess. Blizzard really needs to either bring down druids a bit, or give the lacking healers specs some more utility somehow. I guess part of the issue is that resto druids toolkit just works, it's simple with just a few spells they really need to cast but all of them are really powerful and work in most situations.