Personally, the order I put stats for WHM, is Piety > DET - SS > Crit
Crit is not a terrible stat on WHM, and those saying as much are misguided. However it isn't dependable either and you can largely make up for a lack of Crit with strong Det builds that have full dependency.
A lot of DET can also turn a Cure I into something rivaling a "Cure II" and it's more dependable.
SS, largely is closer to the core of a WHM of being a strong AoE healer. Being able to cast two very potent AoE heals within the time of casting one heal with a chance of very high potency and which being better, is largely determined by the fight in question.
Fights like Titan HM or EX or Leviathan/ Ramuh, SS has a stronger weight then Crit because you have a lot of continuous damage going around to everyone.
However fights like T6 and T9, where damage is more focused on one target, Crit picks up quite a lot and does have a slight edge over SS as a third stat.
I also never quite understood this argument of over-healing, and that being the reason why you shouldn't get Crit. I've tested all of the WHM builds back in the day even when things like Titan HM/EX were solo healed. If your pulling Aggro, one of three things is true.
Either :
A) Your just spamming big heals (Medicas) for no reason.
B) Your tank is just really bad. You don't need a superb weapon to hold aggro from a smart healer. However if your healer is not being mindful of there aggro, theres really nothing a tank can do about that. You can have a tank with full 110, and a healer being stupid with heals can still pull aggro if allowed. If your pulling aggro from using Cure I or Cure II, that's either a very undergeared tank, or a very bad one.
C) Your not properly memorizing fights and using Shroud of Saints to the best of it's ability.
Piety after a certain point is also useless with proper use of Shroud of Saints. We don't run into raids with 0 bards. You will have a Ballad whether you directly need it, our your other healer does, or your SMN does. To cap Peity simply because it's good for sustainment, while true, is not an overbearing difference when you have a Bard in your party. It's not to say this shouldn't be your strongest stat because it should in your builds on a WHM, but to sacrifice everything for it, I don't think is a solid strategy either where you have opportunities for more potent heals. If your looking to Solo heal everything in the game forever and always, then yeah Piety should be capped. However, your largely always gonna have a ballad and a second healer in the mix unless your running some unorthodox method.