It doesn’t stop CC, but if you are CC’ed under the effect of Slithering you still keep on blocking the damage, so at least it has that going for it.
I don’t think the cool-down of Slithering would have been much of a problem, because you only want to cover the down-time of Bunshin, which is 20 seconds duration (on a 49 seconds cool-down). Which means that Slithering will easily cover half of Bunshin’s damage penalty duration but in practice it will be higher because you will use a few seconds beforehand to deal damage and you can cover the rest of the time with hiding or just kiting.
Slithering wouldn’t have to be used all the time either, just whenever it is necessary, so in the end it would be a pretty reliable way to help on dangerous situations.
On a very high CON build, I think it is worth at least a try (and a little bit more with Stabbing’s buff to Magic Defense). But on a 1:1:1 build, I don’t think it will be, but you could still try it out and see what you find.
