It has a MR save, which means a lot of units will escape. If it hits, it still does only 1 armor negating damage. That gives you a 1 + 2d6 damage roll versus a 0 + 2d6 protection roll. So you'll need a good damage roll to seriously hurt the enemy. Once you're surrounded by enemy troops, some are bound to get hit every turn, and soul vortex really shines there.
Actually Max, lifeless units cannot be damaged by drain life or soul vortex. There is no life to drain. They are also immune to life for a life blood magic spell.
__________________
"War is an art and as such is not susceptible of explanation by fixed formula."
- General George Patton Jr.