I don't think Guardians are really that good unless you're trying to slaughter hordes of goblins. Most of the time I just dump them into my magic pile.
In fact, I would think they're slightly overpriced...I mean, standard 2-cost units have 2 melee attack and 2 health, with some ability. At least, that's what I think the baseline is. But Guardians have only 1 attack value, and the ability only makes them do a guaranteed 1 hit. Compared to an average of 1 and 1/3 for other 2-cost units, not forgetting that they often have other abilities (e.g. Berserkers - Assault, Smashers - more health, Priests - healing, Stalwart Archers - range + potential increase in dice, etc.).
I mean, it's always nice to be able to be completely sure of things, but the chance of a Warrior missing totally is only 1/9, about 10% of the time. Fine, it might be game-changing if you miss, but if it's really that important, an SotP can easily guarantee the hits. Or if you're arguing about health, a
Berserker has the same health as the Guardian, costs the same, has the same dice as the Warrior, and an ability that discourages crowding to a certain extent.