Technically, Lightning is even better than Freeze + Shockwave for most creatures. Cheap, fast, requires only 1 card in hand.
Lethal CC (Kill a 5hp creature) is weaker and cheaper than Instant Kill CC (Kill a 499hp creature).
The design of the card is to have more certainty than poison but be slower than poison. It is more certain than poison but is faster than poison. This is the major problem.
Finally, new players will have no way of knowing their Blessing can save their creature. This is the 2nd problem.