There are just some glaring annoyances with AST that people would like different, and in their zealotry people forget that AST isn't that weak on the healing front at all. I don't blame them though, I've seen people only ever using CO on AoE cards, people only ever using AoE cards to begin with, people using Time Dilation on a halved potency card, you name it. Some of the skills aren't as clear in how they should be used and it is easy to get stuck in the mentality that CO/TD = solely card extension.
At least with TD, it becomes a really interesting option to toss a blank Bole on the MT and extend your HoTs with the bole. Boom, pretty good mana-saver right there. Or you could wait and extend only an Enhanced Balance for damage. It doesn't have the comfort of a fairy free-healing, Medica 2's superior range or a disjointed AoE heal (Cure 3/Whispering Dawn), but we have other comforts like lower base manacosts. Comfort can be added without severely affecting potencies, too.
Other obvious stuff is Royal Road baiting (double/triple Ewer/Spire), Spear being really situational (and fairly useless using its own RR buff), Shuffle giving the same card, interactions with Noc shields being lackluster compared to Diu HoTs, Spire being really situational, arguably not needed the way SE's TP design is going. But after giving it second thoughts, stuff like 'let's make CO also have an AoE heal like Assize!' would make any skilled AST blatantly overpower any WHM. Tackle Nocturnal potencies, comfort and reliability, don't tackle global/Diurnal potencies.
On topic: don't get baited into the mindset which number crunchers or fanatics lay out for you too much. Most of them would grill you for even thinking about using Spread on a Bole or a Spear. A skilled AST makes the most out of the situation and doesn't expect the situation to magically convert to how you want it. Test around with your abilities (especially interactions between them) and see how it fits you. If you struggle with a situation, try a different combination the next time. Sometimes, you'd be surprised how useful it can be to never even Royal Road or Shuffle a card, other times you want to RR every second/third card. Practice and make it your own.