Objective-C

Swift's interactions with Objective-C and strict compiler rules make it difficult to work with legacy code. Type casting issues, particularly converting between String and NSString/AnyObject types, and constant unwrapping requirements complicate development.

Developers transitioning from Objective-C find Swift's compile-time type enforcement restrictive and frustrating initially, as it prevents patterns they're accustomed to (like dynamic typing and loose type checking). The strict requirements feel like they 'hold you back' during the onboarding phase.