It's mostly that they have the least ways to deal with sudden crunch time for healing. When left with minimal GCDs, they'll struggle the most. This matters less and less as the gear curve catches up but it stands out earlier in the tier. Alexander 2 being a bit more lax may make it less of an issue but time will tell there.
Alternatively, AST is eventually the best healer for the tier when it comes to farm status period when gear lets them catch-up healing wise and then their buffs for speeding up fights.
I guess I've never been in a situation like that before, not on AST or SCH. I mean, we aren't clearing as promptly as we used to but still.
It's worth noting nobody's saying AST is non-functional, just that bringing an AST is going to be more difficult than bringing the two non-AST healers, and jobs should never be balanced such that one of your options is just flat-out the worst at its role. AST has a toolkit that more or less functions, but will never be considered as-is for early progression because of its shortcomings. And later in content it doesn't stop being a little harder to have an AST, it's just that the content difficulty has decreased relative to gear, so you can get away with a little more GCD starvation.
I mean, I fully intend to live the hilarious AST/WHM disasterfest dream in alt stuff assuming all 8 of us are ever online at the same time. But there's no question that it'd be flat out easier to do all of this if I made my alt a SCH, and it's not a case of class synergy so much as one job coming up short. People mimic the world-first groups in part because the world first groups will gravitate towards things that are most advantageous for the content; if people are approaching things primarily to clear, going with the most advantageous comp makes sense. For more laid back stuff and later clears, it doesn't matter, and people should play what they want as long as everyone knows they're in for potentially a little more struggle. But it not mattering doesn't make AST not inherently worse off than the other two healers (much like being able to clear everything with a PLD doesn't mean PLD's not worse off than the other tanks.)
The thing with card RNG is that it's more beneficial to speedkills and less beneficial when you're just trying to make enrage; you need to have the DPS to kill the thing even if every card you get is meh.
I don't disagree per se, but when someone says a class is inherently worse but either can't put their finger on why or stops paying attention at "this class is worse", bad things happen.
I appreciate the serious discussion about it regarding the comparative GCD limitations vs the off-GCD skills of the other healers (by far the most objective and compelling argument I have ever heard on the matter), but I am also unsure of how much that actually matters in a meta where one of the healers is flat out expected to put out as much DPS as possible for worldfirst clears while the other is expected to be able to hold their own as much as possible to provide those windows of opportunity.
Yet for some reason (and this is just an example), while the comparative lack of Off-GCD abilities would definitely mean a WHM would be better off in that scenario, what about the scenario where the very tangible, passive extra DPS from AST secures a kill that wouldn't have happened otherwise, because that extra DPS just wasn't there?. That would theoretically mean in a situation where a SCH's whiffs would have meant a kill (and your SCH is absolutely going to whiff, cause that is an inevitability of healer acc right now), the AST buffs would secure said kill, yea? Is this situation less rare than AST's less-healing GCDs causing a wipe?
The same question could be said of SCH's current ACC causing some serious fluctuation in DPS output in high end raids when Miasma/Aero is going to inevitably miss: why is that less of a big deal than AST's card stuff? What about when the SCH doesn't whiff but the extra AST DPS is still there? That's more DPS than any other comp! In a meta where DPS is so important to clears. It's just really hard for me to understand, it almost seems hypocritical.
Again, I don't know, and I'm not trying to blame anyone who's come to a conclusion already, but I really like to understand these things: if they are more based around bias towards a certain class (which is fine, they are clearing and I am not) or if it's based around actual data I don't have. I know world firsts tend to hide stuff til after they safety clear so that their competitors won't mimic their ideas with better execution, but it's been a long time since then and I still haven't really seen anything. ]
And holy crap this post went on way longer than I thought it would.