I had this case recently: Sending a job application, so wanting to check what my LinkedIn actually says (I don't use or even update LinkedIn regularly). Now LinkedIn thinks my login looks suspicious and sends a confirmation email. (It does that nearly every time the rare cases I log in, probably because I delete cookies.) But the mail doesn't arrive. My email provider is usually very reliable, but later I learned that just in this moment they experienced multi-hour delays. While this was not a magic link it shows that any login requiring a quick email delivery can fail in the worst moment.