I've been bouncing ideas around quite a bit lately in regards to the state of SCH, and healers as a whole. Things SE could do to improve the feel, making each healer unique and distinct, duty design so that playing healers if more fun, etc.
The one conclusion I had come to in the end, is that healers are on a fundamental level, at least with how FFXIV treats them, nearly impossible to fix.
The reasons are thus:
First of all, it's impossible to scale healing between differing combination of skill levels of healers and the rest of the party. The purpose of healing is twofold. The first is to undo unavoidable damage caused by the enemy. Heal the tank after a tank buster. Heal the raid after a raid-wide AOE. The second is to undo mistakes. Someone stepped in the bad? Heal them.
The problem about these is that they are fundamentally different issues that are covered by a single mechanic. Of course, it's next to impossible to separate these two.
Either way, designing for these two factors is extremely difficult, as they both burden the healer equally, but the factors which decide that are completely separate. Unavoidables are easily planned and the healer's expected skill can be accounted for it based on the content's difficulty. But the latter? The difficulty can be adjusted to a certain degree, but the difficulty the healer faces is more than 75% dictated by the skill level of the party members.
This is a variable amount which cannot be predicted by the neither the developers nor other players. And it's very difficult to actually observe in the duty itself by anyone but the healer and the one making the mistake. Even then, unless if the healer is especially experienced, understanding how much mistake damage should be considered permissible is quite difficult to establish.
The second reason is that no matter how much forced healing an encounter has, the skill of the player directly works against the healer's level of involvement of the encounter. As damage comes both in unavoidables and avoidables, there is a limit to how much unavoidable damage can be sent to the party even if the healer has infinite MP to spend on healing. The reason is that if the healer has 100% uptime for doing its described job just to keep the raid going, there is no leeway to fix avoidable damage if it happens.
But if the healer doesn't have 100% uptime doing its described job, then its left having to do a different job during its downtime. This comes in the form of DPSing, as healers have no other use for their time. Raid design also accounts for this fact, despite class design not doing so.