You'll have to explain this; the old Mac APIs were a turd. I'm not sure how they could be failure beyond the fact that they are so new that many developers aren't comfortable with the Cocoa framework. The framework itself is elegant, and the underlying Unix architecture is practically the definition of elegant from a developer's standpoint. I develop for a lot of different platforms, and MacOS X Jaguar is actually very nice if you use the native APIs and POSIX libs (instead of the API bridges like Carbon, which are a bit gross).
There have been many issues - perhaps the biggest is the fact that the Apple tools no longer work consistently across different platforms. I have to keep separate versions of source for different OS versions. It shouldn't be that way. Jaguar also broke graphics manipluation (NSImage, and NSBitmapRepresentation) big time. Spent months chasing down bugs when the Apple release Notes for Jaguar concede that there was a change and ("This shouldn't affect anyone!") Ha! They are getting way too sloppy.
They have major security holes that the OS system will not install on many systems listed by Apple as being supported, the graphics drawing is extrmemly inconsistent across Apple hardware; simple code compiled under the new tools behaves differently on different hardware; compiler optimizations are completely broken; the 10.2 server is 25% stable as in previous verions; the mail app in 10.2 won't reliably download my mail. None of these problems existed on 10.1.5. On top of that Apple Developer Support doesn't havea clue - their responses to bug reports indicate that they don't even read them and its clear they've never tested this OS on over half the machines they say support it.
Its the worst release of OS X since the second public beta. I've been working with it since before public beta.