Surecast - Ultima's airship crashes (the purple circles) and Twintania's Fireballs both interrupt casts a majority of the time, for two examples on Surecast. I think at least one version of Garuda may have an interrupting "ultimate" type of attack as well, but I can't recall at the moment.

Blizz II is better than Ruin spam in any clumped-up AoE situation as a SCH, as 50x2 = 100 potency > 80 potency from Ruin. If I'm bothering to DPS (hence Cleric), I believe it's only proper that I pack Blizz II as a result, since SCH dps is all about small-pack AoE.

For fun, it's worth noticing that while SCH doesn't get Holy, when all dots (the Bio-Bio II-Miasma trio, Shadow Flare, and Miasma II) are on a set of mobs, that's 110 potency per tick from the trio + 25 from shadow flare + 10 from Miasma II = 145 dot potency per 3s. Add 50 potency to that, per mob, and you're approaching Holy's potency per GCD while all the dots remain in effect.