For me the problem with mail-in votes is that they are (in many jurisdictions) allowed to come in long after the in-person voting is closed, and the preliminary

For me the problem with mail-in votes is that they are (in many jurisdictions) allowed to come in long after the in-person voting is closed, and the preliminary results are annouced. So it creates the space for manipulations, where you count the in-person votes first, and, if the score is close, then a week after the election day half a million of mail-in votes mysteriously comes in and swings the vote one way or another.