Some folks absolutely nailed it. Cleric and Shaman perform differently in different eras of the game in group content.
As an SK/SHM backbone myself, i've tried using a cleric and they do alright in groups. Up to the 60s/70s or so (tlp land) a cleric is probably "better" in terms of raw healing, and the shaman doesn't have as many tools to support a tank unless they are conservatively pull/tank one mob at a time. Shaman start getting their renewal and recourse lines in the 80s or 90s and keeping those at 100% uptime makes a huge difference, plus at 115 (tbl era) wen they have 90% uptime of dissident, there is a lot of rolling passive healing happening tog ive the shaman rbeathing room to slow, dot, take a nap, whatever. the 90-100+ game, shamans have so many tools for passive rolling heals, plus near instant patches (reckless, union, vp click stick, intervention x2..) they can solidly offer a lot to a group.
I also keep a pocket druid that i've wanted to replace for another dps, because in all of my parsing while the SK and SHM account for 80-85% of all group healing, that 15% the druid does has been crucial insurvivability for my playstyle. But that's because I do not believe in CC and pull 4-10+ current er amobs at once and just rip through. the shaman shows some weakness there as a sole healer/slower trying to keep a tank alive with that much damage incoming and trying to slow everything. the druid patching there makes it survivable, and a cleric main with sham heals might be a better solution in all honesty.
it boils down to playstyle, competence, and the era, IMO