Lead Casing helps an individual Bloon be more protected against low-damage attacks, for a high cost. However, if the opponent's Monkeys have Armor Piercing, the investment can be wasted unless they're on cooldown or have no more Armor Piercing sources left.
Mass Lead Production for half the cost can give all of your Bloons Armored 15, making it almost always a better choice in any deck that you'd run Lead Casing in.
This cost and two charges makes this card very easy to brick with.