Thats an exciting interaction, but not one that will go too far
(Outpost Siege, Citadel Siege, and Palace Siege are bombs in Sealed Deck!) Speaking more specifically, they depend on exactly what else you could or should be doing with your mana at that point in the game
All these cards have relatively high casting costs, probably by design, and this one is particularly steep
Against aggro decks, you often side out the Monarch plan anyway, so having Weapon over Fiery Cannonade is always better to leave the board clear
This last ability has become very popular, as players combo it along with other cards such as Scurry Oak, creating infinite creature tokens and putting infinite +1/+1 counters on Scurry Oak
Well, it has happened to me, and often left me uncertain whether my skepticism is justified or if Im overly cautious