I'll talk about SCH as it was my main.
Positives: Seraph looks cool, more mitigation is the direction I'd like to see of SCH, so it is good they got more in the way of shields. The storylines are always good.
Negatives: Too much downtime in general, I'd probably favour weakening some heals in favour of mitigation. And there's little to do in the down time. Ergo I get bored playing it. Healing is too easy, DPS too monotonous. I don't have to put much thought into what I'm doing.
Suggestions for improvement:
Rely more on mitigation. weaker pure heals versus AST or WHM, but good mitigation tools make up for it. Kind of like an evolution of 2.0 SCH, but more suited to today's mode of play. I figure SE may have their reasons for removing our DPS toolkit and I'm not sore in the sense I wanna go big deeps! Because that's still a thing people can do, it's just less interesting to. I am willing to compromise and say maybe if we want to move away from a deeps focus, I'd like to see healer roles, in general, gravitate to more of a support mentality. AST is already on its way there. Specifically for SCH, I think replacing their DPS toolkit with debuffs would work for the job and could include mitigation as a part of it. The 2.0 DPS rotation included debuffs, I'd be okay if they were brought back but without being a part of a DPS rotation.
I'd also like to see even experienced healers to be more engaged with healing outside of EX and Savage content, because we have to level, we do roulettes and so on. One proposal I have is to have a trait learned for when you hit current content, for example, in ShB it could be learned at level 71. When healers get balance for new content it seems that lower content has felt less engaging. Let's say hypothetically we could do a 20% flat nerf to heals and regain that potency at level 71. Healing would need to be more engaged at lower levels. I'm not saying this would be the "ideal" solution, but I think one way it could be approached without necessarily rethinking how they've designed the job for new content. I am sure better ideas could be made.
In talking about making healing more challenging, there is a trap here to simply make stuff hit harder without as much thought behind the process.
The great thing I feel about the dev team is that they increase difficulty in content through mechanics and add penalties for failure greater. An example of what I felt to be good healer mechanic design was the Titan Ex fight, a SCH/WHM set up was great, because the SCH would focus the tankbusters, and mitigate the flurry of damage coming in from Titan whilst the WHM could keep everybody's health topped up. And at times the tank's health would be lower before Mountain Buster hits, so a SCH and WHM could work together. I think how healsets respect mechanics or how mechanics respect healsets can help them feel engaging.