And this is why I think the goal of a cross-platform toolkit is misleading. If your language/library/compiler make it really ergonomic to conditionally target code for a platform, then you get the benefit of keeping your business logic in one place, and the pain only with testing the application.

Usually this is not the case, since interfacing with Cocoa is so different from Win32, for example. However, one shouldn't conflate that difficulty with the difficulty of testing an app across platforms.