Less boring part

Evidently, each healer's MP regeneration is roughly equivalent. There's some variance but it's all within reason; these are all numbers you can work with.

However.
Using Lilies is a potency loss. 3 Lilies + a Misery are 4 GCDs, worth 900 potency. Comparing this to 4x Glare III, you're losing more than a Glare's worth of potency overall. This is fine at present, since WHM's movement options are very limited, but with 1.5s casts, WHM will no longer need to rely so heavily on Lilies to move around. Since they're also a loss, and not DPS neutral, they will likely lean more heavily on the oGCD kit to heal, making Lilies somewhat obsolete (barring very long movements that Swiftcast couldn't cover).
If you took a look at the boring part, you're aware of the implications of this. WHM has little in the way of regenerating MP (or negating MP loss), since the new Thin Air applies to 2 GCDs every 2 minutes, rather than the current 5. This is fine when you're using Lilies, and your MP is comparable to everyone else's. But if you're not, your MP regeneration is much worse, meaning you'd have to change your gear or play suboptimally to keep up, neither of which is a desirable outcome.

There's another less-than-ideal consequence to this. As-is, the best possible thing for a WHM to do is to spam Glare. Meaning, many players will default to this and simply not heal because it's "not what they should be doing". Whether they're correct in saying this is heavily dependent on context, but, one way or another, this point of view is completely justified by the way the class is designed.

The way I see it, making the class this linear simply makes it uninteresting on some level. If you want to play the optimal way, you have no choice between healing and pressing Glare. You just press Glare. And, more broadly, it means WHM's would be less fun to coheal with, as you already know they can help a very limited amount before they start losing potency.

Some possible solutions

1) Make Misery damage-neutral. 4x Glare's worth of potency, instead of the current 900 (1200 would also obviously be acceptable and is perhaps a nicer number). This immediately and completely solves the problem and it becomes a non-issue. Perhaps Rapture's healing potency would have to be reduced in turn, I'm not sure.
2) Make Misery an off-global. This would mean that you'd only be spending 3 GCDs on Lilies before using your Misery, and as such, the point of comparison is 3x Glare's potency, in which case it compares much more favorably. This is a solution that works, but I feel they would be less likely to implement, as they generally make these big finisher moves GCDs.
3) Bring back old Thin Air. This is more of a band-aid fix, and only addresses the MP regeneration part. WHM would still lose potency by healing and the other issues stemming from this persist.

Thank you for reading.