There has been much discussion about the dangers of fraud, lost ballots, extreme delays, etc, which could be associated with a large-scale vote-by-mail policy. There is another danger in vote-by-mail which I haven’t seen much discussed, and that’s the danger of voter intimidation.
If someone is voting at home, then it’s not really always a secret ballot. Someone: an aggressive spouse, a ‘helpful’ minister or activist or neighbor…may exercise a subtle or not-so-subtle pressure. Particularly when one candidate (Trump) is considered socially unacceptable in many circles, I don’t think this is a trivial danger.
In the election held in Germany after Hitler was already in power, closed voting booths were retained, but a large sign in front of them stated: “All patriotic Germans are proud to show how they vote.” There were people there to observe who went into the voting booth and who did not. So almost everyone voted at a table in the open…and guess how almost all of them voted?