That's not really a fair comparison: Windows I/O completion ports are a proprietary standard that is part of the NT kernel, while POSIX AIO is just a user-space libc extension that was added later on (as far as I know, none of the kernel-space posix AIO proposals got merged in). It's easier to get things right when you defined your own standards, at the cost of portability.

But even then, you might argue that that completion ports are less than ideal: you incur a lot of complexity in the file system drivers, while e.g. solaris/BSD offer similar functionality without that burden.

I'm not familiar with BeOS at all, but browsing through Haiku's API documentation I can't really find much. What exactly were you referring to?