Basically, as others have said, Fire Tornado is more of a gamble than anything. In some cases, you might kill the exact units you want dead. In other cases, you'll just smack the same lvl 1 four times (which was meant to die for an SS anyways) and accomplish nothing.
In total, it's far too random to be totally game changing 75% of the time. If it's leaving you open for iczers, then you either A) Don't have many units when it was activated to begin with. B) Had low HP units (say 30 or less), and were unlucky enough to have 4 such units get hit. There's other damage grims that can potentially do better damage (Judgement Hammers, which is pretty much death to 2 separate enemy units, Ball of Flame, which can annihilate a field of great spirits or greatly damage a rush file etc), so Fire Tornado simply can't make the cut as you gain such cards.