My main issue with Swift, at least initially, was how much of a moving target it was/(is?). It's been in public use for under five years and it's already on it's 5th major version. I'd learn it to a certain degree, start working on something else, then come back and find that a lot had changed while I wasn't paying attention. That said, the difficulty in iOS development stems from getting a good understanding of the core libraries, and thats much easier to do in Swift than in Objective-C (unless you're already familiar with that language or coming from something like Smalltalk).