More than viable. It's competitive. The problem lies in inexperience. A LOT of people began playing with the expansion without prior knowledge to mechanics and the ast buffs furthered the problem despite being an amazing potential booster for people who already knew how to play the job. Mch and drk have these same sorts of traits- inexperienced players not utilizing the abilities to the fullest. All 3 of these jobs have cd's that essentially need to be used on recast to get their full benefit. The inexperience comes in when people become hesitant/unaware of how to use their abilities to the fullest. With ast the use of luminiferous aether is the big one. Pre and post buff I've seen complaints about ast enmity generation which is quite literally a non issue with proper cool down usage. The same goes for mp management. I've noticed this to be the hurdle for a lot of drk's as well with not using blood price correctly. TLDR reread your tool tips and learn to maximize your abilities to their fullest and ast is a phenomenally versatile healer with the best raid utility, second to none when used correctly. They don't necessarily replace whm or sch but in most current content it's best to combine ast with one or the other to speed things up.