If someone else reduces your chosen foe to 0 Hit Points or you dont choose a foe, this card has no effect
Spider Spawning is significantly more difficult to deal with than cards like Bloodline Keeper (which can get smoked by a Brimstone Volley, bounced by a Silent Departure, hunted down with Prey Upon, etc)
You can pay one black mana and sacrifice six creatures that all share the name Shadowborn Apostle to search your deck for a Demon creature card, put it directly into play, and shuffle your library
That downside is often mitigated by a drain effect youve got online anyway
Madam Mim in particular can be trickier for new players, requiring the player to banish one of their own cards to bring out the shapeshifting witch - if you dont plan ahead, you might end up discarding one of the cards you needed
For some reason, if a Blue card does this, it tends to get banned, but when a Green card does it, it's no big deal