I don't get it, what's the problem with Swift, iOS 7 compatibility and iPhone 4? I've been working on an app for iOS 7+ devices for a few months and everything

I don't get it, what's the problem with Swift, iOS 7 compatibility and iPhone 4? I've been working on an app for iOS 7+ devices for a few months and everything is working ok. The pains I'm facing with Swift are far from compatibility with iOS (debugging is hell compared to Objective-C).