I mean, it SOUNDS like an easy question to answer...but what's the answer?
1) Do only GCDs matter? SCH has around 2x the oGCDs as WHM and they tend to have more indirect effects (less direct healing, more mitigation or resource generation/consumption), but people often don't factor those in as disruptions to the Glare/Broil/Malific/Dosisspam paradigm. Do oGCDs count.....or do they not really count?
2) Is it based on interaction of kit abilities? For example, Dosis generates Kardia. Spending Aetherflow generates Faerie Gauge. Plenary Indulgence modifies AOE GCD heals. Are these examples of something being hard/complex, or are they not?
3) Does availability of backup tools make a Healer easy? Most of WHM's oGCD kit is more or less mimicked by some of their GCD heals, allowing them to cast those (at a damage loss) in cases the oGCDs are on cooldown. SCH has the most limited spamable healing kit in the game, and none of its oGCDs directly replicate the effects of Adloquium or, really, even Succor - even though Adlo (outside of specific situations) and Succor are considered backup tools, they actually have different use cases as SCH's oGCDs don't actually have the ability to generate those effects. Is this complexity or perhaps not?
4) Is complexity only a measure of their damage kits? WHM's damage kit is arguably more involved than SCH, but it doesn't feel that way to a lot of players. So is SGE's. AST has the most slimmed down damage kit in the game, and the most spam nuke casts (Malific) of all the Healers in a given clear. So on the surface, that would make SCH the second least complex Healer and AST the least complex. Yet the community views them as the hardest and most complex. So clearly damage kits aren't a measure of complexity...but then damage kits are always brought up as a component of complexity, even though they seem to be trumped by...something?...else.
5) Is pre-planning a component of complexity? If so, how much? Remembering to use Earthly Star 10 seconds before a raidwide vs using Afflatus Rapture right after the raidwide, are these really very different in complexity? Some people might replace "remembering" with "planning", but in that case, how is planning out when to use your Raptures less complex than planning out when to use your Earthly Stars? Saving and using Recitation + Adlo + Deployment Tactics is more buttons with an arguably tighter application window, so is it more complex? What about Lilybell, another prepatory heal? Or Asylum, a slower heal but that also modifies other abilities to heal for more allowing combinations? How do we measure and rate the relative complexity, and why is one case of "press 1 button" considered more complex than another?
.
In short, we talk a TON about simple/easy/braindead and hard/complex/galaxy brain stuff, but how can we actually quantify that? How can we break that down into what SPECIFIC things go into that evaluation, and which trump which when it comes down to it - e.g. AST has the simplest damage rotation but arguably the hardest oGCD one with some kit interactions and required pre-planning for optimization, so if we see AST as the most complex healer, that would suggest that damage rotation is trumped by the other metrics.
As we saw in the "Healers Then And Now" thread, sometimes these things can be a bit misleading. Only by actually breaking it down was it immediately apparent that WHM's damage rotation is actually more complex than SCH's, something that almost anyone (me included) would have said was the opposite before actually looking at it in that depth.
I get part of it is a "feel", but "feel" is different from person to person. Some people may see something simple (on paper) to be challenging to execute, others may feel something complex is child's play, and still others may see a distinction between "complex" and "clunky", relating the latter more to "bad controls" than to actual thoughtful mechanics and skill expression.
For terms oft thrown out, it might pay to have a more concrete way of looking at them.
.
I get it's actually a complex question, and is some combination of the above (and probably some other things, besides), but we should have more than a truly vague notion of something if it's so important.