Mw are Gods in WSG. i would put them as God tier or even S-Tier since (*Note that the list is aimed mostly towards PuGs.)
MW have insane healing a Enveloping Mist alone heals for around 1k over 7 sec, Soothing Mist is around 1,1k over 7 sec and Vivify 330+ which you can spam while Soothing mist is up. i would say MW have the highest possible HPS at lvl 19.
They also have ALOT of mobility which is really handy. even have a taunt you can compind with roll to catch rogues etc in stealth.
Mw also have one of the highest Bursts at lvl 19. 1000+ damage in 2 gcds, 1200+ damage in 3 gcds and then a super strong ranged attack aswell Crackling jade lighting is a channeled ability which deals instant damage on use which means it can be spammed for even more damage/ticks and its often the Main damage ability in pugs and its super good to kill steal
and depending on your race you can even grab a stun or something like that.
With all that compined i would rate it alittle higher than holy priests and shit
MW has fine healing, good damage, however, you're more easily punished for your damage than the healers listed above MW. Thing is, if you don't bring a lot of utility in today's bursty meta (shield, cc, slows, mobility, dots, safe damage), then your class will always be below those that do. MW is good, just not as good disc and shaman. It's a really good third to compliment a disc that can provide safe catch-up mechanics through shields, and then let your high HPS as MW shine to top people safely and quickly.
A few things that makes those healers so good in current meta:
-Panic buttons (unleash life, disc shields, Holy Word: Serenity)
-Crowd control (Psychic scream AoE, so it's crazy strong when on the defensive (gauntlet etc.))
-Ranged damage that's good and doesn't necessarily require a big investment (dots for instance)
-"On-the-go" healing (penance while moving/shields, renew, healing totem (crazy good btw), riptide)
-Mobility (Ghost Wolf is sick)
-Earthbind Totem
-Mana management (MW tends to oom way faster than those I put in above tiers)
And the list could go on for a bit longer, and with more detail, but this should be enough to go on.
MW isn't bad, it's really good, it's just that the other healers bring more to the table. Also the order in the tier segments are not rankings. It's just to put them in the tier where they belong, so because holy priest is above MW, although they are in the same tier, it doesn't mean holy priest is better. They're just in the same tier.