Level 60 AST/SCH here.

AST can heal fine in any content before raids, so long as the group is competent and doesn't get hit by too much avoidable damage. If the group is taking more damage then they should be, AST goes OOM VERY quickly and lacks the emergency buttons to save multiple party members if a situation turns sour.

In exchange for this inherent weakness, AST has random card buffs which generally do very little to help the party overall. Even the hallowed expanded RR + Balance combo is only 5% more damage for 15 seconds, and that's garbage, considering the difficulty and randomness of setting it up in the first place.

I think they should just improve the AST's emergency options, because a hybrid healer just isn't going to work in a game with a duty finder where the healer is going to have to heal people who make mistakes a lot or wind up getting kicked (as some ASTs have already complained about).