More from the analysis...

... One of the most important reasons we use paper ballots in elections is to protect our elections against the possibility of computer hacking or error. If the optical-scan voting machines miscount the votes, we can recount the paper ballots by hand. To save the trouble and expense of always recounting by hand, we can do a risk-limiting audit of a well-chosen sample of the ballots. In those recounts or audits, teams of human auditors inspect the marks made by human voters. And even if the mark wasn't made by a human, suppose it was a touch-screen ballot-marking device (BMD) that marked the vote onto the paper, then at least (in theory if not in practice) that vote can be inspected and verified by the voter before they cast the ballots.

This principle assumes that the votes are printed onto the paper ballot in a form that the voter can read. But some BMDs print the vote onto the paper in the form of a barcode or QR code, in addition to printing the candidate selection in human-readable form. The voter can verify what the human-readable text says, but that's not the mark that will be read and counted by the optical scanner. This leads to serious problems, as described in the "Votes Encoded in Barcodes" section of the experts' letter on Suggested Principles for State Statutes Regarding Ballot Marking and Vote Tabulation.

So therefore: Votes should not be marked on ballots in the form of barcodes or QR codes.

But there's another use of barcodes or QR codes in paper ballots that's actually quite useful, reasonable, and effective. That is, to mark a ballot style.

In a typical jurisdiction, such as a county, there are hundreds of voting precincts. Not every precinct has the same contests and candidates on the ballot. Some of the precincts are in one town, some in another, so there will be different choices for Mayor on one ballot than on the other. Similarly, different precincts might be in different legislative districts. That is, different precincts will have different ballot styles, and each ballot style will have its own set of contests and candidates to vote for. The county election administrator must deliver to each local precinct the appropriate ballot style. ...