To be honest, I'm kind of disappointed in how AST was handled considering they were supposed to "fluidly replace either healer" and "compatible in any party".

The stat bonuses they provide in parties. AST and SCH both give PIE even if AST is in "WHM Stance". I've already seen the fighting between AST and SCH for spots in Alexander raids starting, considering no one wants to lose WHM's +MND buff (it's about a 26 mind difference which is pretty noticeable on a healer), that just leaves SCH and AST fighting for the second healer spot.

And the lower potency is pretty much what people consider the nail in the coffin. People assume lower potency = weaker heals = more damage = dead party. I heard that AST's shields don't double when they crit like a SCH's (I don't know if it's true). But if it is, then that has basically made every AST undesirable (especially for progression) if they can't provide the same mitigation as SCH on top of sharing SCH's PIE buff. Weaker shields break faster, which results in more damage, which means more healing, and I don't think offering the other healer Ewer Cards is really gonna make up for that, lol.

"Fluid" and "compatible", but not really.