Red Dead Redemption is an open-world Western for iPhone and iPad, following John Marston as he hunts former gang members to protect his family. The mobile edition also includes Undead Nightmare, which turns the same frontier into a zombie outbreak, so there are two distinct single-player stories to settle into.
Riding between towns gives way to conversations and shootouts, with on-screen controls dividing movement, aiming, firing, and taking cover into separate actions. That's a lot to coordinate when several enemies start shooting. GamesRadar's account of playing the Android port describes exactly that problem, particularly against the zombie crowds. It's a useful description of the workload, although it can't establish how an iPhone performs or whether a particular accessory works with it.
I'd treat the controls as the deciding factor. In a launch discussion on r/iosgaming, several players struggled with touch aiming and stagecoach driving. October and November US App Store reviews ask for Backbone support. One names the Lightning model, a useful reminder to check the accessory itself before assuming it solves the problem.
A substantial Western campaign on a portable screen is the draw for iPhoners willing to spend time finding a workable control setup. The single-player scope is the attraction; multiplayer is absent.
