Collective Unconsciousness also provides a mitigation bonus, so while it's less versatile it also tends to have a stronger effect at the time of application. Contextually, there are times where CU is great and other times it'll just feel like a flop which is fight dependent.
Healing over Time themselves are a snap shot at the time of application and maintain that for the entire duration of the HoT. In the case of Regen and Diurnal Aspected Benefic, they'll get bonuses from Divine Seal, Synastry, and Fey Illumination for the entire duration of the HoT. Likewise, if the target(s) happens to have an Infirmity debuff or popped their Convalescence / Mantra, they'll also receive the penalty / bonus for the entire duration of the HoT as well.
Take into account that Collective Unconciousness, Indomitability, and Assize are all ability based heals. As every single healing buff/debuff in the game only affects magic/spells, these particular abilities aren't affected by Infirmity or Healing Potency booster (thus making Lustrate still an incredible anti Infirmity tool.
Those puns >>; lol /slowclap
With that being said, one can possibly argue that Assize's 10% MP restoration is balanced by ASTs 10%+ reduced MP costs, but I certainly won't argue SCHs ability to generate MP is absurd to the max (but that's more of a discussion at how powerful SCH is than AST/WHM MP tools). There in one more reliable way to generate MP on AST if you're willing to spend the CDs on it - using Celestial Opposition to increase Luminious Aether's duration, giving the AST two more ticks of MP. Better than hoping for an Ewer and also gives you slightly more MP than Ewer too.
It's actually not immediately obvious, so don't feel bad about that. XD You aren't the first person to not notice that and you certainly won't be the last one either. I would personally prefer to keep the 20% reduced MP cost versus any other effect, but that's a personal preference. Also, don't get me started on Aspected Benefic Diurnal vs Regen because I quite a few people can't distinguish the two either ; ;