Resto Shaman has had garbage throughput compared to other healers since SoD released, and this was made substantially worse in P3, where AoE healing is the only healing that actually matters in the raid.
Here are some simple comparisons with my shaman and the other healers in my raid. For reference, I’m at about 360 bonus healing. I’m not comparing it to Paladin, because it’s a horde issue.
Chain heal hits for about 750, 375 and 188, for a total of 1313, with a cast time of 2.5 seconds. This results in 526 HPS.
Healing Rain hits for 90, on five targets, with a cast time of 2 seconds and a duration of 10 seconds. This results in 2250 HPS per cast, and 450 HPS while active.
Circle of Healing hits for about 750, on five targets, with an instant cast time. This results in 2500 HPS per cast using the gcd, and 625 HPS per cooldown.
Wild Growth heals for 281, 253, 227, 200, 174, 148 and 121 for a total of 1404, on five targets, with instant cast time. This results in 4680 HPS per cast using the gcd, and 1003 HPS per 7 second duration.
Efflorescence heals for 106, on five targets, taking two gcds to cast, and with a duration of 15 seconds. This results in 2650 HPS per cast, ignoring the continued hot and swiftmend, and 530 HPS while active.
It is very clear that Shaman AoE healing is not even in the remote vicinity of being competitive. The other benefits that resto shaman brings are a shamanistic rage, though this is usually one of four, mana spring, and mana tide. Mana spring and mana tide are both very weak compared to shamanistic rage.
There is an obscure fringe benefit of healing rain being able to apply the 25% armor buff fairly consistently on tanks, but this is not relevant in sunken temple, as physical damage on tanks is never an issue. It is a very strong buff with no actual use.
Core issues of chain heal are that it does poor healing, as can be seen by it not even outhealing efflorescence, it only healing 3 targets, the healing falling off very sharply on consecutive targets, and the cast time being very long. The long cast time often leads to other healers already healing a target before the chain heal cast is finished. Perhaps the worst thing about chain heal is its reputation, though. Chain heal is strong in era, and somehow this makes people believe that it is strong in SoD as well, even though this is demonstrably not the case.
Healing rain, while having adequate HPS, has two major issues. First, there is no ground effect that indicates the range of the spell. Nobody knows where the heal is placed, so nobody can move into it intentionally. Second, the cast time should be instant, even without the 3p bonus. Having lower HPS than efflorescence, even though efflorescence has a clear ground effect and higher uptime, is ridiculous.
A simple fix for this would be a change to tidal wave, to grant a cast time reduction to chain heal, as it does for healing wave, making chain heal consume tidal wave instead of proccing it. This decreased cast time would also make resto shaman spend slightly more mana, which is desirable, as they currently might as well not even have mana costs.
Further, a waist or feet slot rune could be added, as these slots currently do not have resto shaman runes. These could be used to decrease the healing reduction for consecutive targets of chain heal, and increase the number of targets.
Alternatively, a more interesting approach would be to introduce waist or feet slot runes that grant new AoE healing abilities. Healing Stream Totem could be replaced by a mid-cooldown totem that does AoE healing, similar to wild growth. Another option would be a rune that makes chain heal spread riptide to its targets, instead of consuming riptide. This would help increase resto shaman AoE healing throughput, and give them the forgotten p2 runes.