I have used shroud wearing guys with Ark many many many times in MP and they have been in many veeery long battles and didn't get affected even once, so I am 99% certain that shroud is working.
Ark? Only MR/eyeless and looks like it doesn`t work on lifeless beings (mechanical men, iron dragons etc.), but it needs check
I think, Ark is more useful like some kind of mobile hidden powerful priest. Combo "stealthy priest + holy sword + Ark + some kind of deadly enchantment" works very good