If you're using it to usurp a nation from a distance, you can get similar results with nature, air, death, or blood spells. From my experience imprint souls is one of the more powerful versions, but it's SO expensive. You could teleport a thug onto the province for 2 astral pearls.
If you're using it for the units, again it's SO expensive. You get a handful of unarmored fanatics and a mad priest, but if you have a bless strategy then your nation obviously has far better sacred units than you could get from Imprint Souls.
Yeah, I'm quite ticked, I forgot to change the spell before the turn ran and was unimpressed with the results. Now, if it was a random sacred unit of your nation, that'd be pretty cool.