The question is how things go when someone doesn't feel like playing ball. With email and not-signed-for letters there's no reasonable way to prove that the person saw the content. "Spam probably ate it." "It must have gotten lost in the mail." And so on.
So hand-served (and signed letter) subpoenas remain relevant for when people are dodging you. The fact that many people do respond to email subpoenas doesn't relate to whether non-respondents can be charged for their failure.