There's a lot more to it than just potency.
an astro is pretty comparable to a white mage in playstyle
they have spells similar to cure, cure II, medica, and tetragrammaton
and then they have a spell that is like a mana inefficient renew or a really mana inefficient and weak but instant cast adloquiem
and a numerically weaker medica II or succor
It's a decent kit but then you compare to just say a white mage who on top of that has divine seal, presence of mind, benediction, assize, cure III fast stoneskin, and several traits for mana efficiency
play them both and you'll see for yourself it's like your playing a kiddy version of a real healer
and the comparison to scholar is not even really existent they just made scholar a beast in the amount of emergency healing and utility they bring
But the healing is actually passable for what we currently have so it comes to the astro utility
synastry is actually very good with the only gripe that it doesn't work for overhealing at all
disable is very bad, if they reworked it to be "next skill or spell" so the invisible auto attacks during skill casts didn't eat it, it would be ok
celestial opposition is kind of just "ooo pretty sparkles"
time dilation is good if you can manage to get proper cards up
and the cards
spire and ewer are basically useless outside of warrior and dark knight
bole is usually the only card you draw when you don't need it and never around when you do
the spear is garbage for anything but spreading it to make your luminiferous aether cooldown quicker
and the balance and the arrow are amazing (if you manage to get them)
but the issue is you can shuffle around cards for like 2 minutes sometimes just to get one good buff out of it or you may get nothing but spear for an entire fight or you can just draw balance over and over again for an entire fight. It's so random with like 4/6 of the cards being nothing but fodder, if they were all actually really good all the time it would be a different story. Having to choose the right target for each awesome card is a good thing VS "blech i keep getting useless crap!"
so it boils down to choices of
one of the two healers with strong healing, good mana efficiency, good damage potential, and reliable utility
or
the healer with weaker healing, bad mana efficiency, bad damage potential, and unreliable utility