Not much to this spell, if you want a decent number of good melee creatures then go for it, though griffins are probably a more versatile creature to summon if possible. Buffing your existing stacks is probably a better idea early on in the combat. Though Griffins are more versatile, Unicorns are slightly cheaper, and thus, they will be able to summon more Unicorns than they would Griffins. Therefore, if your hero happens to have both spells, check the numbers before deciding which to cast, unless other factors make one spell preferable to the other.