Hashbrowns' tier is prob right.
As someone who's played disc since 4.3.4, it may be as powerful as it's ever been. Fast and powerful heals, great AoE cc. Holy is also really good, but disc feels practically invincible.
Rsham is really really fun. Your heals aren't as powerful as disc (though if you get in "the zone" you can get close) but you get amazing utility. Windshear is wonderful, and the slow totem is great in WSG and EOTS, and flame shock as a way to keep stealthier out of combat is great. I rolled a new Pandaren sham for the extra stun. Sure it lost purge in Legion, but it's still great. Is there another heal spec in the bracket with so much utility? So far, despite being a lifelong disc, I've been having so much fun with rsham the past few days.
MW is harder to get right. The heals are slower. It's okay for single target, but the current cast-time/dot-time/heal-per-cast is somewhat less than ideal. You get a lot of healing out if, but it takes longer to setup, and by then it may be too late. If you want to heal a team fight with another healer you can be useful, but a MW can't solo-heal the way Disc/rsham can.
Hashbrowns and I have played disc+rdruid combo in several games, and it can work. If he says it's fun, I'd take that as a good endorsement.
I'm ashamed to say, but I've only really heal'd a handful of games in retail wow, so I can't really comment on it.