In 2005, former President George W. Bush asked his Justice Department to determine whether signing legislation passed by Congress with an autopen is constitutional, given that the
U.S. Constitution dictates in Article I, Section 7 that presidents "shall sign" bills into law. The government's lawyers
determined the president "need not personally perform the physical act of affixing his signature to a bill he approves and decides to sign in order for the bill to become law" and
may "direct a subordinate to affix the President's signature to the bill."
U.S. presidents have used autopens since Thomas Jefferson; the first to use an autopen to sign a law was Barack Obama.
www.snopes.com
So, all we really have is a DOJ opinion on the use of an auto pen.
It should go before SCOTUS by way of charging one of those pardoned to settle the matter for good and all.