Its not really as simple as this level of reductiveness. Yes AST is a very good healer. Yes it can happily hold its own in the current EX primals. Yes it does need looking at, as there are some stupid failings with it.
The disparity in power between diurnal and nocturnal is obvious at the moment. Bismark EX no point in being in anything but diurnal, and its a joke to solo heal this as AST (with a cohesive group), Ravana its either or stance, take your pick. The fact that Time Dillation interacts very strongly with diurnal and not nocturnal is another indicator of some interesting design decision on this job that need looking at. It should have perhaps increased the potency of any Nocturnal shield already on the target rather than extending the time the shield lasts on the target.
The card system need tweeking in potencies, lengths of buffs and interaction of royal road with next card drawn perhaps, but there are may threads on this forum that go into really good detail about this so i wont here.
Celestial Opposition is plain bad. It comes across as a move that was too powerful as originally conceived, changed last minute due to its power, they couldnt think of anything better to give us so they went "well they kind of have holy without the stun, so why dont we just give them the stun at lvl 60 as a completly separate, useless underwhelming move". For a lvl 60 move, this should extend the buffs on a target by a minimum of 15 secs, but thats just my opinion (it should do smething far better than this, but if it is a buff extention, it needs to be better than time dillation).
AST is a good healer, but i suspect that it got a lot of last minute nerfs before release due to the devs being scared of making it too overpowered. We were told in live letter etc that the stance change had a long cooldown so we had to be careful when to use it; we got stance locked in battle instead. We got told their lvl 60 ability was powerful and had to be used carefully to full effect; we got stun, one that doesnt effect raid mobs and a 5 sec increase on buffs (aka a pile of rubbish). We got told that the AST brings a lot of benefits from cards and that its probably better to be drawing cards to buff others rather than dps'ing ourselves, suggesting that cards > our dps as AST; we got a fairly lackluster buff system with the cards, situationally very powerful, but still lacking in areas.
However, no matter the above, AST is a very powerful healer, it does however need looking at.
