To elaborate on yebyen's answer: these shortcuts can be handled in different layers of your stack. By default, ^U will be handled by your terminal emulator, and it will only delete the buffer, it won't remember what you've deleted. However, programs can ask the terminal emulator to pass ^U through to them so they can handle it themselves. This happens if you use libeditline (or libreadline), and these libraries implement a per-application buffer.
You can test this yourself. Type some text, then ^U, then ^Y. This works because your shell uses libreadline or libeditline. Try this again in a "dumb" program like cat, or a shell without line editing like /bin/dash. ^U works, but ^Y does nothing (well, it lets you type ^Y in as much as you want).
No, I don't think so. Not unless you are accustomed to typing your password directly into the terminal prompt. Kill buffers don't pass from one application to another and they don't all behave the same in every context, either.
(For example, if I type some stuff and then ^U to kill the password buffer at an SSH password prompt, typing ^Y after that seems to put the whole process in the background. Not sure what that's intended to do, but subsequent ^Y presses don't retrieve the password into the shell prompt...)
