One big reason why I am not in favor of mail-in ballots is rejected ballots.
I was an election judge for Belgrade Township before they went to mail in. We had one of the best systems in the country. Voters filled in a paper ballot and inserted it into a machine that checked it for mistakes and, if none, tallied the vote. If the voter made a mistake, the machine rejected it, the voter handed us the rejected ballot and was given a new one to fill out.
At the end of the day we would have a pile of rejected ballots. Size of the pile depended on how many voted.
In California, 102,428 mail-in ballots were rejected in the March primary elections.
In addition to the mistakes we found in Belgrade Township, mail-in ballots are prone to more rejects, such as missing signature, signature not matching what is on file, lost in mail, timely filing, etc.
In a tight election, rejected ballots could make the difference.
Would you want your vote to not be counted?