Completion notification requires committing a (potentially cache-hot) buffer for an operation that might not complete until some time in the future. With readiness notification you only need to have the buffer ready when you know you will use it.

Also, on an high performance poll/epoll/kevent based system you only need to poll cold fds, while you can do speculative direct read/writes to hot fds, so no need for extra syscalls in the fast case.

That doesn't mean that completion notification doesn't have its advantages, especially when coupled with NT builtin auto-sizing thread pool, but it is not strictly better.