The main problem with her is that support-wise, she needs too much farm to be useful. Her shield is basically pointless except for the fact that it's a shield, meaning you need to farm and stack AP for it to be good. It doesn't help that the only other utility it has is damage which ALSO needs you to stack AP. As a support.
Her heal is none too impressive base-value wise either, and again, unlike, say, Soraka or Nidalee's heal, it has nothing other than the fact that it doubles as a nuke, which ALSO requires AP. If you DON'T have AP, all it is is a 135 + 5% (Essentially 1/20th of someone's missing health) of missing health heal, at max level. Which is, well, garbage, considering it doesn't give you anything else. Sure it's AoE, but the fact that you're probably healing people that don't need healing at the moment limits the effectiveness of it being AoE, at least in comparison to the 80 armor from Soraka's heal, 60% ASPD from Nidalee's heal, 20 armor/mr team-wide buff from Sona's heal, and teamfight reset from Janna's.
And to top it all off, you need Mantra for the heal to even apply. Or the damage from the shield. And she only has 3 abilities.
The problem is that Riot treats her like a normal champion and gives her 3 normal abilities, instead of 3 above-average abilities to compensate for her lack of ult. In fact, you even need that stupid mantra mechanic to even PUT her heal and shield on the level of an average normal ability, because they are just god awful without mantra, period. A mediocre cone AoE nuke with crappy scaling on a 9 second cooldown and a shield that just... is a shield with no extra utility.
And on top of all that, she has no CC except her rather meh spirit bonds. She's a support that NEEDS farm to be even vaguely ON PAR with other supports.