Confession: I detest mail-in voting. Imagine the biggest asshole you know. Now think of them "helping" their partner or kids or employees vote "the right way", multiplying the power of their asshole ballot.
Voting can't be free unless it's anonymous. Mail-in voting isn't anonymous. I know the reasons why people like it, but I think they're outweighed by the inherent loss of privacy in marking your choices where there could potentially be others watching.