Summon: Yojimbo and Summon: Ixion are probably the nastiest to keep around, as both of them have a Chapter I that lets you exile something, and if youre doing that every single turn, your opponents are going to be in trouble
If the target creature is an illegal target when Waltz of Rage tries to resolve, it won't resolve and none of its effects will happen
Effects that switch a creature's power and toughness apply after all other effects, regardless of when those effects began to apply
You should absolutely play it
But then again, thats a world in which no opponent draws more than one card per turn, and it has been a while since I last played a game like that
Enter the Infinite I love this Gatecrash card, conceptually