Looking at the rule book, it feels like there may be something missed. There is a sidebar that discusses keywords used in invocation effects. Purify is mentioned. It is to cast out or harm otherworldly beings. This brings an interesting question. Should that damage automatically be sacred? It feels like it should, but many purify invocations are earth , and earth invocations have a standing use of opp to make damage sacred. So maybe not? For example, Jade Strike. Should the damage from Jade strike be considered Sacred, or only if you spend an opp?

I'd say keep it to the Opp spends, or the couple other ways to add Sacred damage. Remember Sacred bypasses ALL forms of resistance on Otherworldly, period. This means if you're doing all Sacred damage all the time, there's really no purpose for some of these entities to have Supernatural DR.

If not everything is Sacred, this means a GM can present different challenges. A Ghost for instance is insubstantial and has Resistance 10, meaning they're much more of a narrative challenge unless you have access to Sacred weaponry - while maybe a physically manifest Gaki only has a little bit of resistance to either physical or supernatural, so you can have a decent number of minions say, come boiling out of a Ghost Ship, then your characters without Sacred can still be effective, but if you have a free Opp you can squeak out being slightly more effective.

Sacred quality for weapons makes a lot of things irrelevant and if only one character in the party have one, it makes the others feel useless against some enemies. It is way too strong and makes some enemies that should be difficult to destroy a joke. Why sacred goes thru plate armor again?

anyway, just throw that rule in the substantial pile of trash design. The Sacred quality needs a rework. When one quality can fully bypass all kinds of resistances in the game for both tainted and otherwordlies, thats like, plain stupid.
If you GM this game, you will realise how much of a crutch this garbage quality is.

If you have nay idea how to nerf it, let me know. Right now I just decide what sacred goes thru and what it doesnt (ex, I might it let go thrue Hide/Skin armor, but not Plate). Which helps, a bit.