The Teir system they give is basically the strength in progression raiding at the highest level, there is representation from all classes at the highest level meaning they're all viable, but not all of them carry the same strength in progression raiding.Disc/Sham/Paladin have always been pretty decently high rated progression healers(with the exception of shaman being DOODOO in bfa for like a good half the expack because the thoughput nerfs were so bad)Disc/Paladin have raid CDs(Spiritspell/Barrier/Aura-mastery) / tank Externals / they add damage in-addition to all of this that is apart of their kit so they don't sacrifice HPS like shaman needs to due to its lack of passive healing.Shaman has huge thoughput this teir and one of the strongest raid CDs(spirit link) and a CD to pop every mechanic, wither it be earthen wall totem/ a charged cloudburst/healing tide empowered super rain/Spirit link, they have an option for every mechanic.all the healers are viable for HPSing, what separates the S teir is just the utility and damage they offer which is a huge boon during progression raiding. most groups would run 3progression-teir healers/1 though-put likely druid/MW/nightfae-holy priest