In my extensive experience with storyboards, XIBs & NIBs, I've found it to be incredibly easy to unintentionally change something that has a profound effect on how that particular view works. Frankly, with iOS storyboards or XIBs, I'd be much more comfortable only having engineers handle visual tweaks.
Recent example - went into a storyboard, changed a constraint, must have accidentally clicked on the wrong thing at one point, and the next time I ran my app, there was a faint detail disclosure icon in the middle of my view controller. It was absurd - somehow, unintentially, I had redirected a push segue onto a UIImageView. No clue how it happened, but it took me a good 10 minutes to diagnose it.
IB is just way too buggy to trust non-engineers with UI work, IMO.
Recent example - went into a storyboard, changed a constraint, must have accidentally clicked on the wrong thing at one point, and the next time I ran my app, there was a faint detail disclosure icon in the middle of my view controller. It was absurd - somehow, unintentially, I had redirected a push segue onto a UIImageView. No clue how it happened, but it took me a good 10 minutes to diagnose it.
IB is just way too buggy to trust non-engineers with UI work, IMO.