Force of Vigor is premium against Jeskai Artifacts, capable of destroying two artifacts at once (like a Mox Opal and an Emry, Lurker of the Loch target, or hitting Urza's Saga before its third chapter) and can be cast for free by exiling a green card. The extra Outland Liberator gives another cheap artifact-destruction body to answer their Goblin Welder/Goblin Engineer recursion engine or a resolved Agatha's Soul Cauldron. Shadowspear and Sylvan Safekeeper do little against a deck that isn't attacking your life total with creatures or targeting your permanents with removal, and Scythecat Cub's slow clock doesn't keep pace with their combo-artifact engine.