I unlocked AST immediately upon HW release and powered it to 50 on the first two days to begin the HW content.
The only dungeon I had any issues with healing was Stone Vigil which I jumped into at 41. And this was due to a tank(DRK) wearing full STR accessories and gear. There were no wipes in this instance however I found that I had much less time to DPS or use card buffs. This was pre-Ewer/Spire changes, so I found myself using Benefic II quite a lot and with only Luminiferous Aether to restore MP, it was rough.
I then did all the HW content as Astrologian (I already had WHM and SCH at 50) and experienced very little issues in the 51-60 story quests and instances.
While AST can DPS reasonably well (not as well as a WHM or SCH), the MP cost for Gravity and Malefic II are quite high. The job is what you make of it really - I personally prefer healing on AST for things such as EX roulette because it feels a lot more involved - however when it comes to things like Alex 3/4 I do prefer SCH due to it's reliability. I do hope it gets buffed further in the coming patch because the Astrologian is a great concept that sadly does have numerous shortcomings.
When compared to WHM and SCH around the level 40-45 mark, you're missing things such as;
*Divine Seal - 30% healing increase for 15 seconds(WHM) *Rouse - 40% healing increase for 20 seconds(Faerie)(SCH) which leads to-- *Eos/Selene - Free 300 potency heals from Faerie. AoE Regen/Healing potency increase/Magic defense, and AoE Esuna/Fey Wind. *Holy - AoE damage and stun(WHM) *Adloquium - 300 potency heal with barrier(SCH) *Regen - 150 potency heal over time for 21 seconds.
Comparing these to what AST has at this level range; *Card buffs *Essential Dignity - Your OHSHIT heal which increases in potency the lower the recipient's HP%. 40 second cooldown *Aspected Benefic(Diurnal) - 18 second Regen with a potency of 100. Also 190 potency standard cure. *Aspected Helios(Diurnal) - AoE heal potency of 140 and a Regen potency of 50 for 30 seconds.
The mistake I made at 50 was that I started to live in Nocturnal Sect because of the 5% healing potency it gives. While this may sound appealing - the barriers aren't as potent as Scholar's and Regen is more MP efficient.