I wouldn't be so quick to say that Healing Rain has poor value for mythic+ I'm looking more forward to that than I am the new Golden and I'm here being a proponent for the new golden. People need to understand how situational our toolkit is for Mythic+, when you're going to heal, where you're going to heal, and how you're going to use your heals drastically change depending on group composition, affix, and Dungeon. 95% of the time I'm running an Unleash, Guidance, AV, Echo, Wellspring Build. Probably fairly uncommon but I'm not the cookie cutter type all I can say is try it and you'll love healing M+ so much more
As for Legendaries I use Velen's and Uncertain Reminder the Intellect from these two pieces is all but unrivaled and Velen's is just too damn good.
My biggest complaint is that Healing Rain has a cooldown, it would be much less frustrating if it behaved like Efflorescence in that it would be able to be recast at will.