Viable means they can all clear the content, which is true.
Are they balanced? Hahahahahahahahahahaha.
WHM is AST without cards now. Heck, AST's healing toolkit alone beats out WHM's. Spells are generally cheaper, Potencies for non-basic healing spells aren't far off, and they actually have access to worthwhile mitigation via Collective and Disable. Once you include their cards? Bye bye WHM.
Benediction and Cure 3 ain't a pile of chocobo dung in comparison.
All healers have pretty close potency/GCD when it comes to single target DPS. I believe WHM and AST are actually slightly higher. SCH is just more capable of creating DPS windows and their sustain is so high that they can keep going long after a WHM has bottomed out on mana. Most of their DoTs also don't require accuracy, so even an unmelded SCH can contribute a sizeable chunk of DPS with 3/4 GCDs every so often. Their DoTs also have pretty long durations, so dropping out of Cleric's Stance to Adlo is going to hurt their output less than a WHM who has to recast Aero 2 after every 4 GCDs.
It's less that SCH does too much, and more that DPSing doesn't hurt it anywhere near as much as say, WHM (expensive DPS skills that all require meeting the accuracy cap, combined with a large portion of their mana management being tied into their healing procs).
How could they fix WHM? Couldn't tell you for sure. But the class is in dire need for some actual love and an identity outside of "big healz" (which isn't even that true). Buffs to stoneskin (lower cast time & increase HP percentage to bring more in line with Adlo) and giving native raidwide mitigation (like Disable/Soil) would make it compete better in the realm of healing toolkits. Decoupling their resource management from their healing toolkit and moving it elsewhere (or giving similar procs to DPS skills) would go a long way to improving their sustainability.