My midrange Necro deck has 1x Unstable Zombie for versatility. It doesn't have the power of Brute, and mostly Venemous and Plague are better for their niches, but it does get some play.
That said, I have a much more aggressive Brute squad book, and a much more controlling book, which are both better overall books than the mid-range.
Unstable is not so bad that it's almost unplayable like many creatures, but it's not strong.