Unison Spaulders are gimped in raid healing because Soothing Mist can't be expected to do hardly anything.
Shelter of Rin is insulting considering the mechanical issues with using Sheilun's Gift. "Do you miss Uplift yet Mistweavers?"
Petrichor Lagniappe is the "you're almost as good as a Druid" knockoff.
Ovyd's Winter Wrap only activates on overhealing, then spreads a short-lived healing buff to only 2 random people, so if the planets align it's mediocre at best.
Leggings of The Black Flame is one of the legendaries I actually enjoy but not everyone is going to enjoy RNG in a healing environment. Most of the legendaries in the game should be designed with a utility centric focus like this one.
Ei'thas, Lunar Glides of Eramas are similarly as awkward as Ovyd's: You get a crappy buff if you happen to be spending your time not actually healing.
Eye of Collidus the Warp-Water happens to be the easiest to get the full benefit of. It's not terrible since it's the most versatile legendary for raiding and dungeons that isn't Prydaz or Velen's, but it really only brings Life Cocoon in line to what it probably should be shielding for anyway.
Listening to the other players in my guild complaining about their legendaries and how they're not viable without them and I'm sitting here thinking how most healing legendaries are crap and uninteresting. Velen's synergizes with Essence Font in terms of stats and the overhealing effect. It's funny that Prydaz is best for throughput and the shield is more utilitarian than most of the other legendaries we have.