For Raid healing:
Resto shaman sustained output is probably #1 on fight’s where you don’t have to move very much and the raid is clumped. This falls off as mobility becomes more important and the raid spreads out but in general Shaman are excellent group healers. We are a bit less efficient when it comes to tank healing though but we can still use them to bounce a lot of chain heals through the melee group.
For Mythic+ healing:
With only 1 hot/instant heal (unless you talent for unleash life - and you usually need undulation more) this can make high level mythics tough. The key there is to realize that in order to carry your weight you’re not just a healer - you are a CC/utility machine. We can’t compare to other classes if we just try to heal all the damage here - we have to prevent more of it with our AOE stun, super short interrupt, slow totem for kiting, etc. Earth elemental for reaping waves is awesome.